High yield ratio-type, hot rolled high strength steel sheet excellent in formability or in both of formability and spot weldability, and production thereof

ABSTRACT

A high yield ratio-type, hot rolled high strength steel sheet excellent in both formability and spot weldability, containing not less than 5% of retained austenite, and a process for producing the same are provided. The steel sheet contains 0.05 to less than 0.15% by weight or 0.15 to less than 0.30% by weight of C, 0.5 to 3.0% by weight of Si, 0.5 to 3.0% by weight of Mn, more than 1.5 to 6.0% by weight of Si and Mn in total, not more than 0.02% by weight of P, no more than 0.01% by weight of S, and 0.005 to 0.10% by weight of Al, the balance essentially being Fe, and is composed of three phases of ferrite, bainite and retained austenite as a microstructure, and having a ratio (V F  /d F ) of ferrite volume fraction (V F ) to ferrite grain size (d F ) of not less than 20 (not less than 7 in case of 0.15 to less than 0.30% by weight of C), a volume fraction of retained austenite having grain sizes of not more than 2 μm being 5% or more, a yield ratio (YR) of not less than 60%, a strength-ductility balance (tensile strength×total elongation) of not less than 2,000 (kgf/mm 2 .%), an enlargement ratio (d/d o ) of not less than 1.4 (not less than 1.1 in case of 0.15 to less than 0.30% by weight of C), and a uniform elongation of not less than 15% (not less than 10% in case of 0.15 to less than 0.30% by weight of C).

TITLE OF THE INVENTION

High yield ratio-type, hot rolled high strength steel sheet excellent informability or in both formability and spot weldability, and productionthereof.

TECHNICAL FIELD

The present invention relates to a hot rolled high strength steel sheet(plate) with a high ductility and an excellent formability or excellentformability and spot weldability, directed to use in automobiles,industrial machines, etc. and to a process for producing the same.

BACKGROUND ART

Prior Art

Due to keen demands for lighter weight of automobile steel sheets andsafety assurance at collisions of automobiles as main backgrounds,higher strength is required for steel sheets. However, workability isrequired even for the high strength steel sheets, and steel sheetscapable of satisfying the requirements for both strength and workabilityare in keen demand. Heretofore, dual phase steel (which will behereinafter referred to as "DP steel") comprising ferrite and martensitehas been proposed for hot rolled steel sheets for use in the field thathas required a good ductility. It is known that DP steel has a betterstrength-ductility balance than those of solid solution-intensified,high strength steel sheets and precipitation-intensified, high strengthsteel sheets, but its strength-ductility balance limit is atTS×T.E1≦2,000. That is, DP steel fails to meet more strict requirementsin the current situations.

As means capable of meeting the requirements in the current situationsto attain TS×T.E1>2,000, it has been proposed to utilize retainedaustenite. For example, Japanese Patent Application Kokai (Laid-open)No. 60-43425 discloses a process for producing a steel sheet containingretained austenite, which comprises hot rolling a steel sheet in atemperature range of Ar₃ to Ar₃ +50° C., retaining the steel sheet in atemperature range of 450° to 650° C. for 4 to 20 seconds and coiling itat a temperature of not more than 350° C., and also Japanese PatentApplication Kokai (Laid-open) No. 60-165320 discloses a process forproducing a steel sheet containing retained austenite, which comprisesconducting high reduction rolling of a steel sheet at a finishingtemperature of not less than 850° C., at an entire draft of at least80%, a total draft of at least 60% for final three passes and a draft ofat least 20% for the ultimate pass, and then conducting cooling to 300°C. or less at a cooling speed of at least 50° C./s.

However, these conventional processes are not preferable in practicefrom the viewpoints of energy saving and productivity improvement,because of retention at 450° to 650° C. for 4 to 20 seconds during thecooling, coiling at a low temperature such as 350° C. or less, highreduction rolling, etc. Furthermore, the workability of the steel sheetsproduced by these processes is at TS×T.E1<2,400, which would not alwayshave fully satisfied the level required by users. That is, steel sheetshaving a higher TS×T.E1 (desirably more than 2,400) and a highproductivity process for producing such steel sheets have still been indemand. On the other hand, in view of the actual formability, not only agood strength-ductility balance, but excellent uniform elongability(stretchability), enlargeability or hole expansibility (enlargeabilityinto a flange shape), bendability, secondary workability, and toughnessare also required. Furthermore, in the service field of these steelsheets, spot welding is more and more used, and thus an excellent spotweldability is also required. Still further, not only a higher tensilestrength, but also a higher yield ratio (higher yield strength) isrequired from the viewpoint of strength assurance.

That is, the field of actual application can be considerably broadenedby satisfying these requirements at the same time.

(Problems to be solved by the invention)

The present invention provides a hot rolled, high strength steel sheethaving an excellent workability, containing retained austenite and beingcapable of attaining TX×T.E1≧2,000, which is over the limit of the priorart, and also a process for producing the same. Furthermore, the presentinvention provides a hot rolled, high strength steel sheet having anexcellent formability (strength-ductility balance, uniform elongability,enlargeability, bendability, secondary workability and toughness), ahigh yield ratio and an excellent spot weldability at the same time andalso a process for producing the same.

DISCLOSURE OF INVENTION

To solve the above-mentioned problems, the present invention uses thefollowing means (1) to (20):

(1) A high yield ratio-type, hot rolled high strength steel sheetexcellent in both formability and spot weldability, characterized bycomprising 0.05 to less than 0.16% by weight of C, 0.5 to 3.0% by weightof Si, 0.5 to 3.0% by weight of Mn, more than 1.5 to 6.0% by weight ofSi and Mn in total, not more than 0.02% by weight of P, not more than0.01% by weight of S, and 0.005 to 0.10% by weight of Al, the balanceconsisting essentially of Fe, as chemical components, being composed ofthree phases of ferrite, bainite and retained austenite asmicrostructure, and having a ferrite grain size (d_(F)) of not more than5 μm, a ratio (V_(F) /d_(F)) of ferrite volume fraction (V_(F)) toferrite grain size (d_(F)) of not less than 20, a volume fraction ofretained austenite having a grain size of not more than 2 μm being notless than 5%, and a yield ratio (YR) of not less than 60%, astrength-ductility balance (tensile strength×total elongation) of notless than 2,000 (kgf/mm².%), an enlargement ratio (d/d_(o)) of not lessthan 1.4, and a uniform elongation of not less than 15% ascharacteristics.

(2) A high yield ratio-type, hot rolled high strength steel sheetexcellent in both formability and spot weldability, characterized bycomprising 0.05 to less than 0.16% by weight of C, 0.5 to 3.0% by weightof Si, 0.5 to 3.0% by weight of Mn, more than 1.5 to 6.0% by weight ofSi and Mn in total, not more than 0.02% by weight of P, not more than0.01% by weight of S, and 0.005 to 0.10% by weight of Al, and 0.0005 to0.01% by weight of Ca or 0.005 to 0.05% by weight of REM, the balancebeing Fe and inevitable elements, as chemical components, being composedof three phases of ferrite, bainite and retained austenite asmicrostructure, and having a ferrite grain size (d_(F)) of not more than5 μm, a ratio (V_(F) /d_(F)) of ferrite volume fraction (V_(F)) toferrite grain size (d_(F)) of not less than 20, a volume fraction ofretained austenite having a grain size of not more than 2 μm being notless than 5%, and a yield (kgf/mm².%), an enlargement ratio (d/d_(o)) ofnot less than 1.4, ratio (YR) of not less than 60%, a strength-ductilitybalance (tensile strength×total elongation) of not less than 2,000 and auniform elongation of not less than 15% as characteristics.

(3) A process for producing a high yield ratio-type, hot rolled highstrength steel sheet having both an excellent formability and spotweldability, and also having a yield ratio (YR) of not less than 60%, astrength-ductility balance (tensile strength×total elongation) of notless than 2,000 (kgf/mm².%), an enlargement ratio (d/d_(o)) of not lessthan 1.4 and a uniform elongation of not less than 15%, characterized byconducting a finish-rolling of a slab prepared by casting a steelcomprising 0.05 to less than 0.16% by weight of C, 0.5 to 3.0% by weightof Si, 0.5 to 3.0% by weight of Mn, more than 1.5 to 6.0% by weight ofSi and Mn in total, not more than 0.02% by weight of P, not more than0.01% by weight of S, and 0.005 to 0.10% by weight of Al, the balanceconsisting essentially of Fe, as chemical components, in an endtemperature range of Ar₃ ±50° C. at an entire draft of not less than 80%and an ultimate pass strain speed of not less than 30/second, conductingcooling at a hot run table at a rate of not less than 30° C./second, andconducting coiling at a temperature of more than 350° C. to 500° C.

(4) A process for producing a high yield ratio-type, hot rolled highstrength steel sheet having both an excellent formability and spotweldability, and also having a yield ratio (YR) of not less than 60%, astrength-ductility balance (tensile strength×total elongation) of notless than 2,000 (kgf/mm².%), an enlargement ratio (d/d_(o)) of not lessthan 1.4 and a uniform elongation of not less than 15%, characterized byconducting a finish-rolling of a slab prepared by casting a steelcontaining 0.05 to less than 0.16% by weight of C, 0.5 to 3.0% by weightof Si, 0.5 to 3.0% by weight of Mn, more than 1.5 to 6.0% by weight ofSi and Mn in total, not more than 0.02% by weight of P, not more than0.01% by weight of S, and 0.005 to 0.10% by weight of Al, and 0.0005 to0.01% by weight of Ca or 0.005 to 0.05% by weight of REM, the balancebeing Fe and inevitable elements, as chemical components, in an endtemperature range of Ar₃ ±50° C., at an entire draft of not less than80% and an ultimate pass strain speed of not less than 30/second,conducting cooling at a hot run table at a rate of not less than 30°C./second, and conducting coiling at a temperature of more than 350° C.to 500° C.

(5) A process for producing a high yield ratio-type, hot rolled highstrength steel sheet having both an excellent formability and spotweldability, and also having a yield ratio (YR) of not less than 60%, astrength-ductility balance (tensile strength×total elongation) of notless than 2,000 (kgf/mm².%), an enlargement ratio (d/d_(o)) of not lessthan 1.4 and a uniform elongation of not less than 15%, characterized byconducting a finish-rolling of a slab prepared by casting a steelcomprising 0.05 to less than 0.16% by weight of C, 0.5 to 3.0% by weightof Si, 0.5 to 3.0% by weight of Mn, more than 1.5 to 6.0% by weight ofSi and Mn in total, not more than 0.02% by weight of P, not more than0.01% by weight of S, and 0.005 to 0.10% by weight of Al, the balanceconsisting essentially of Fe, as chemical components, at an endtemperature of not less than Ar₃ -50° C., at an entire draft of not lessthan 80% and an ultimate pass strain speed of not less than 30/second,conducting cooling at a hot run table down to a temperature T₁ in arange of not more than Ar₃ to more than Ar₁ at a rate of less than 30°C./second, and from T₁ downwards at a rate of not less than 30°C./second, and conducting coiling at a temperature of more than 350° C.to 500° C.

(6) A process for producing a high yield ratio-type, hot rolled highstrength steel sheet having both an excellent formability and spotweldability, and also having a yield ratio (YR) of not less than 60%, astrength-ductility balance (tensile strength×total elongation) of notless than 2,000 (kgf/mm².%) an enlargement ratio (d/d_(o)) of not lessthan 1.4 and a uniform elongation of not less than 15%, characterized byconducting a finish-rolling of a slab prepared by casting a steelcomprising 0.05 to less than 0.16% by weight of C, 0.5 to 3.0% by weightof Si, 0.5 to 3.0% by weight of Mn, more than 1.5 to 6.0% by weight ofSi and Mn in total, not more than 0.02% by weight of P, not more than0.01% by weight of S, and 0.005 to 0.10% by weight of Al, and 0.0005 to0.01% by weight of Ca or 0.005 to 0.05% by weight of REM, the balancebeing Fe and inevitable elements, as chemical components, at an endtemperature of not less than Ar₃ -50° C., at an entire draft of not lessthan 80% and an ultimate pass strain speed of not less than 30/second,conducting cooling at a hot run table down to a temperature T₁ in arange of not more than Ar₃ to more than Ar₁, at a rate of less than 30°C./second, and from T₁ downwards at a rate of not less than 30°C./second, and conducting coiling at a temperature of more than 350° C.to 500° C.

(7) A process for producing a high yield ratio-type, hot rolled highstrength steel sheet having both an excellent formability and spotweldability, and also having a yield ratio (YR) of not less than 60%, astrength-ductility balance (tensile strength×total elongation) of notless than 2,000 (kgf/mm².%), an enlargement ratio (d/d_(o)) of not lessthan 1.4 and a uniform elongation of not less than 15%, characterized byconducting a finish-rolling of a slab prepared by casting a steelcomprising 0.05 to less than 0.16% by weight of C, 0.5 to 3.0% by weightof Si, 0.5 to 3.0% by weight of Mn, more than 1.5 to 6.0% by weight ofSi and Mn in total, not more than 0.02% by weight of P, not more than0.01% by weight of S, and 0.005 to 0.10% by weight of Al, the balanceconsisting essentially of Fe, as chemical components, at an endtemperature of not less than Ar₃ -50 ° C., at an entire draft of notless than 80% and an ultimate pass strain speed of not less than30/second, conducting cooling at a hot run table down to a temperatureT₁ in a range of not more than Ar₃ to more than Ar₁ at a rate of notless than 30° C./second, and from T₁ downwards to a temperature T₂ in arange of not more than T₁ to more than Ar₁ at a rate of less than 30°C./second, and furthermore from T₂ downwards at a rate of not less than30° C./second, and conducting coiling at a temperature of more than 350°C. to 500° C.

(8) A process for producing a high yield ratio-type, hot rolled highstrength steel sheet having both an excellent formability and spotweldability, and also having a yield ratio (YR) of not less than 60%, astrength-ductility balance (tensile strength×total elongation) of notless than 2,000 (kgf/mm².%), an enlargement ratio (d/d_(o)) of not lessthan 1.4 and a uniform elongation of not less than 15%, characterized byconducting a finish-rolling of a slab prepared by casting a steelcomprising 0.05 to less than 0.16% by weight of C, 0.5 to 3.0% by weightof Si, 0.5 to 3.0% by weight of Mn, more than 1.5 to 6.0% by weight ofSi and Mn in total, not more than 0.02% by weight of P, not more than0.01% by weight of S, and 0.005 to 0.10% by weight of Al, and 0.0005 to0.01% by weight of Ca or 0.005 to 0.05% by weight of REM, the balancebeing Fe and inevitable elements, as chemical components, at an endtemperature of not less than Ar₃ -50° C., at an entire draft of not lessthan 80% and an ultimate pass strain speed of not less than 30/second,conducting cooling at a hot run table down to a temperature T₁ in arange of not more than Ar₃ to more than Ar₁ at a rate of not less than30° C./second, and from T₁ downwards to a temperature T₂ in a range ofnot more than T₁ to more than Ar₁ at a rate of less than 30° C./second,and furthermore from a T₂ and downwards at a rate of not less than 30°C./second, and conducting coiling at a temperature of more than 350° C.to 500° C.

(9) A high yield ratio-type, hot rolled high strength steel sheetexcellent in formability, characterized by comprising 0.16 to less than0.30% by weight of C, 0.5 to 3.0% by weight of Si, 0.5 to 3.0% by weightof Mn, more than 1.5 to 6.0% by weight of Si and Mn in total, not morethan 0.02% by weight of P, not more than 0.01% by weight of S, and 0.005to 0.10% by weight of Al, the balance consisting essentially of Fe, aschemical components, being composed of three phases of ferrite, bainite,and retained austenite as microstructures, and having a ferrite grainsize (d_(F)) of not more than 5 μm, a ratio (V_(F) /d_(F)) of ferritevolume fraction (V_(F)) to ferrite grain size (d_(F)) of not less than7, a volume fraction of retained austenite having a grain size of notmore than 2 μm being not less than 5%, and a yield ratio (YR) of notless than 60%, a strength-ductility balance (tensile strength×totalelongation) of not less than 2,000 (kgf/mm².%), an enlargement ratio(d/d_(o)) of not less than 1.1, and a uniform elongation of not lessthan 10% as characteristics.

(10) A high yield ratio-type, hot rolled high strength steel sheetexcellent in formability, characterized by comprising 0.16 to less than0.30% by weight of C, 0.5 to 3.0% by weight of Si, 0.5 to 3.0% by weightof Mn, more than 1.5 to 6.0% by weight of Si and Mn in total, not morethan 0.02% by weight of P, not more than 0.01% by weight of S, and 0.005to 0.10% by weight of Al, and 0.0005 to 0.01% by weight of Ca or 0.005to 0.05% by weight of REM, the balance being Fe and inevitable elements,as chemical components, being composed of three phases of ferrite,bainite, and retained austenite as microstructures, and having a ferritegrain size (d_(F)) of not more than 5 μm, a ratio (V_(F) /d_(F)) offerrite volume fraction (V_(F)) to ferrite grain size (d_(F)) of notless than 7, a volume fraction of retained austenite having a grain sizeof not more than 2 μm being not less than 5%, and a yield ratio (YR) ofnot less than 60%, a strength-ductility balance (tensile strength×totalelongation) of not less than 2,000 (kgf/mm².%), an enlargement ratio(d/d_(o)) of not less than 1.1, and a uniform elongation of not lessthan 10% as characteristics.

(11) A process for producing a high yield ratio-type, hot rolled highstrength steel sheet having an excellent formability and also a yieldratio (YR) of not less than 60%, a strength-ductility balance (tensilestrength×total elongation) of not less than 2,000 (kgf/mm².%), anenlargement ratio (d/d_(o)) of not less than 1.1, and a uniformelongation of not less than 10%, characterized by conducting afinish-rolling of a slab prepared by casting a steel comprising 0.16 toless than 0.30% by weight of C, 0.5 to 3.0% by weight of Si, 0.5 to 3.0%by weight of Mn, more than 1.5 to 6.0% by weight of Si and Mn in total,not more than 0.02% by weight of P, not more than 0.01% by weight of S,and 0.005 to 0.10% by weight of Al, the balance consisting essentiallyof Fe, as chemical components, in an end temperature range of Ar₃ ±50°C. at an entire draft of not less than 80% and an ultimate pass strainspeed of not less than 30/second, conducting cooling at a hot run tableat a rate of not less than 30° C./second, and conducting coiling at atemperature of more than 350° C. to 500° C.

(12) A process for producing a high yield ratio-type, hot rolled highstrength steel sheet having an excellent formability and also a yieldratio (YR) of not less than 60%, ratio (d/d_(o)) of not less than 1.1,and a uniform elongation a strength-ductility balance (tensilestrength×total elongation) of not less than 2,000 (kgf/mm².%), anenlargement of not less than 10%, characterized by conducting afinish-rolling of a slab prepared by casting a steel comprising 0.16 toless than 0.30% by weight of C, 0.5 to 3.0% by weight of Si, 0.5 to 3.0%by weight of Mn, more than 1.5 to 6.0% by weight of Si and Mn in total,not more than 0.02% by weight of P, not more than 0.01% by weight of S,and 0.005 to 0.10% by weight of Al, and 0.0005 to 0.01% by weight of Caor 0.005 to 0.05% by weight of REM, the balance being Fe and inevitableelements, as chemical components, at an end temperature range of Ar₃±50° C. at an entire draft of not less than 80% and an ultimate passstrain speed of not less than 30/second, conducting cooling at a hot runtable at a rate of not less than 30° C./second, and conducting coilingat a temperature of more than 350° C. to 500° C.

(13) A process for producing a high yield ratio-type, hot rolled highstrength steel sheet having an excellent formability and also a yieldratio (YR) of not less than 60%, a strength-ductility balance (tensilestrength×total elongation) of not less than 2,000 (kgf/mm².%), anenlargement ratio (d/d_(o)) of not less than 1.1, and a uniformelongation of not less than 10%, characterized by conducting afinish-rolling of a slab prepared by casting a steel comprising 0.16 toless than 0.30% by weight of C, 0.5 to 3.0% by weight of Si, 0.5 to 3.0%by weight of Mn, more than 1.5 to 6.0% by weight of Si and Mn in total,not more than 0.02% by weight of P, not more than 0.01% by weight of S,and 0.005 to 0.10% by weight of Al, the balance consisting essentiallyof Fe, as chemical components, at an end temperature of not less thanAr₃ -50° C., at an entire draft of not less than 80% and an ultimatepass strain speed of not less than 30/second, conducting cooling at ahot run table down to a temperature T₁ in a range of not more than Ar₃to more than Ar₁ at a rate of less than 30° C./second, and from T₁downwards at a rate of not less than 30° C./second, and conductingcoiling at a temperature of more than 350° C. to 500° C.

(14) A process for producing a high yield ratio-type, hot rolled highstrength steel sheet having an excellent formability and also a yieldratio (YR) of not less than 60%, a strength-ductility balance (tensilestrength×total elongation) of not less than 2,000 (kgf/mm².%), anenlargement ratio (d/d_(o)) of not less than 1.1, and a uniformelongation of not less than 10%, characterized by conducting afinish-rolling of a slab prepared by casting a steel comprising 0.16 toless than 0.30% by weight of C, 0.5 to 3.0% by weight of Si, 0.5 to 3.0%by weight of Mn, more than 1.5 to 6.0% by weight of Si and Mn in total,not more than 0.02% by weight of P, not more than 0.01% by weight of S,and 0.005 to 0.10% by weight of Al, and 0.0005 to 0.01% by weight of Caor 0.005 to 0.05% by weight of REM, the balance being Fe and inevitableelements, as chemical components, at an end temperature of not less thanAr₃ -50° C., at an entire draft of not less than 80% and an ultimatepass strain speed of not less than 30/second, conducting cooling at ahot run table down to a temperature T₁ in a range of not more than Ar₃to more than Ar₁, at a rate of less than 30° C./second and from T₁downwards at a rate of not less than 30° C./second, and conductingcoiling at a temperature of more than 350° C. to 500° C.

(15) A process for producing a high yield ratio-type, hot rolled highstrength steel sheet having an excellent formability and also a yieldratio (YR) of not less than 60%, ratio (d/d_(o)) of not less than 1.1,and a uniform elongation a strength-ductility balance (tensilestrength×total elongation) of not less than 2,000 (kgf/mm².%), anenlargement of not less than 10%, characterized by conducting afinish-rolling of a slab prepared by casting a steel comprising 0.16 toless than 0.30% by weight of C, 0.5 to 3.0% by weight of Si, 0.5 to 3.0%by weight of Mn, more than 1.5 to 6.0% by weight of Si and Mn in total,not more than 0.02% by weight of P, not more than 0.01% by weight of S,and 0.005 to 0.10% by weight of Al, the balance consisting essentiallyof Fe, as chemical components, at an end temperature of not less thanAr₃ -50° C. at an entire draft of not less than 80%, and an ultimatepass strain speed of not less than 30/second, conducting cooling at ahot run table down to a temperature T₁ in a range of not more than Ar₃to more than Ar₁ at a rate of not less than 30° C./second, from T₁downwards to a temperature T₂ in a range of not more than T₁ to morethan Ar₁ at a rate of less than 30° C./second, and furthermore from T₂downwards at a rate of not less than 30° C./second, and conductingcoiling at a temperature of more than 350° C. to 500° C.

(16) A process for producing a high yield ratio-type, hot rolled highstrength steel sheet having an excellent formability and also a yieldratio (YR) of not less than 60%, ratio (d/d_(o)) of not less than 1.1,and a uniform elongation a strength-ductility balance (tensilestrength×total elongation) of not less than 2,000 (kgf/mm².%), anenlargement of not less than 10%, characterized by conducting afinish-rolling of a slab prepared by casting a steel comprising 0.16 toless than 0.30% by weight of C, 0.5 to 3.0% by weight of Si, 0.5 to 3.0%by weight of Mn, more than 1.5 to 6.0% by weight of Si and Mn in total,not more than 0.02% by weight of P, not more than 0.01% by weight of S,and 0.005 to 0.10% by weight of Al, and 0.0005 to 0.01% by weight of Caor 0.005 to 0.05% by weight of REM, the balance being Fe and inevitableelements, as chemical elements, at an end temperature of not less thanAr₃ -50 ° C. at an entire draft of not less than 80%, and an ultimatepass strain speed of not less than 30/second, conducting cooling at ahot run table down to a temperature T₁ in a range of not more than Ar₃to more than Ar₁ at a rate of not less than 30° C./second, from T₁downwards to a temperature T₂ in a range of not more than T₁ to morethan Ar₁ at a rate of less than 30° C./second, and furthermore from T₂downwards at a rate of not less than 30° C./second, and conductingcoiling at a temperature of more than 350° C. to 500° C.

(17) A process for producing a high yield ratio-type, hot rolled highstrength steel sheet excellent in both formability and spot weldabilityaccording to any one of the above mentioned items (3) to (8),characterized in that the hot finish-rolling initiation temperature ofthe steel is not more than Ar₃ +100° C.

(18) A process for producing a high yield ratio-type, hot rolled highstrength steel sheet excellent in both formability and spot weldabilityaccording to any one of the above mentioned items (3) to (8),characterized in that after the coiling the steel sheet is cooled to200° C. or less at a cooling speed of not less than 30° C./hour.

(19) A process for producing a high yield ratio-type, hot rolled highstrength steel excellent in formability according to any one of theabove mentioned items (11) to (16), characterized in that the hotfinish-rolling initiation temperature of the steel is not more than Ar₃+100° C.

(20) A process for producing a high yield ratio-type, hot rolled highstrength steel sheet excellent in formability according to any one ofthe above mentioned items (11) to (16), characterized in that after thecoiling the steel sheet is cooled to 200° C. or less at a cooling speedof not less than 30° C./hour.

(Function)

As a result of extensive tests and studies, the present inventors havesolved the problems of the prior art and have found a hot rolled highstrength steel sheet having an excellent formability, a high yield ratioand an excellent spot weldability together, and a process for producingthe same.

Firstly, the microstructure of a steel sheet that can meet an excellentformability and a high yield ratio at the same time must be composed ofthree phases of ferrite, bainite and retained austenite, where theretained austenite has grain sizes of not more than 2 μm at a volumefraction of not less than 5%; ferrite grain size (d_(F)) is not morethan 5 μm; and V_(F) /d_(F) (V_(F) : ferrite volume fraction in %, d_(F): ferrite grain size in μm) is not less than 20 (or not less than 7 whenC is in a range of 0.15 to less than 0.3% by weight, because finerretained austenite grains can be readily formed).

In Table 1, their relations are shown, and their points are summarizedin the following items 1 to 3:

                                      TABLE 1                                     __________________________________________________________________________    Microstructure of steel sheet                                                                                                   Bainite, other              Characteristics γ.sub.R V.sub.F /d.sub.F ≧ 20                                                            V.sub.F /d.sub.F ≧                                                               phase than                  of steel sheet  ≦2 μm                                                                   ≧5%                                                                        d.sub.F ≦ 5 μm                                                            0.05% ≦ C <0.15%                                                                 0.15% ≦ C                                                                        ferrite,                    __________________________________________________________________________                                                      γ.sub.R               Strength-ductility balance                                                                    ◯                                                                     ◯                                                                     ◯                                         Uniform elongation                                                                            ◯                                                                     ◯                                                                     ◯                                         (stretchability)                                                              Enlargeability  ◯                                                                         ◯             ◯               (enlargeability into flange shape)                                            Bendability     ◯                                                                         ◯             ◯               Secondary workability                                                                         ◯ ◯                                                                           ◯                                                                           ◯               Toughness       ◯                                                                         ◯                                                                       ◯                                                                           ◯                                                                           ◯               Yield ratio (yield strength)                                                                          ◯                                                                       ◯                                                                           ◯                                                                           ◯               __________________________________________________________________________     ◯ shows a strong corelation                                  

1 Increase in the retained austenite contributes to improvements ofstrength-ductility balance and uniform elongation, and its effect isenhanced by making the retained austenite grains finer. By making theretained austenite grains finer, the enlargeability or the holeexpansibility, bendability, secondary workability and toughness can bemaintained in an excellent level. That is, by making the content ofretained austenite 5% or more and the grain size not more than 2 μm, anexcellent strength-ductility balance, an excellent uniform elongation,an excellent enlargeability, an excellent bendability, an excellentsecondary workability and an excellent toughness can be obtained at thesame time.

2 Increase in V_(F) /d_(F) contributes to improvements of the secondaryworkability and toughness and an increase in the yield ratio through anincrease in the ferrite volume fraction and finer ferrite grain size(d_(F) ≦5 μm).

3 By making the microstructure composed of three phases of ferrite,bainite and retained austenite, that is, by avoiding the inclusion ofpearlite and martensite, the enlargeability, bendability, secondaryworkability and toughness can be maintained at an excellent level,whereby a high yield ratio can be also maintained.

Secondly, in order to contain retained austenite at a volume fraction ofnot less than 5%, as shown in FIGS. 1 and 2, it is necessary to controla Si content to 0.5-3.0% by weight, a Mn content to 0.5 to 3.0% byweight, and a Si+Mn content to more than 1.5 to 6.0% by weight, and makea V_(F) /d_(F) ratio not less than 20, in case of 0.05 to less than0.15% by weight of C, and to control a Si content to 0.5 to 3.0% byweight, a Mn content to 0.5 to 3.0% by weight and a Si+Mn content tomore than 1.5 to 6.0% by weight and make a V_(F) /d_(F) not less than 7,in case of 0.15 to less than 0.30% by weight of C. In order to make theretained austenite grain size not more than 2 μm, it is necessary tomake a finish-rolling ultimate pass strain speed not less than30/second, as shown in FIG. 3.

Thirdly, in order to obtain a best spot weldability (inside-nuggetbreakage=0), it is necessary that a C content is less than 0.15% byweight, a Si+Mn content is not more than 6% by weight, a Si content anda Mn content are each not more than 3.0% by weight and a P content isnot more than 0.02% by weight, as shown in FIG. 4.

Fourthly, in the case that a very stringent surface property isrequired, it is effective to control the heating temperature to not morethan 1,170° C. and a Si content to 1.0 to 2.0% by weight.

Fifthly, in order to obtain an excellent enlargeability (d/d_(o) ≧1.4),it is necessary to make a C content less than 0.15% by weight and a Scontent not more than 0.01% by weight, and it is also effective to addCa or REM thereto, as shown in FIG. 5. In order to obtain a particularlyexcellent enlargeability (d/d_(o) =1.5), it is further necessary to makea C content less than 0.10% by weight.

That is, various combined characteristics required for a hot rolled highstrength steel sheet can be satisfied only by strict component controland strict structure control according to the present invention.

The present inventors have made further studies of hot rollingconditions for obtaining the above-mentioned micorstructure and havefound a process for producing a hot rolled high strength steel sheet.

At first, component control values and the reasons for the control willbe explained below.

Not less than 0.05% by weight of C must be added to assure the retainedaustenite (which will be hereinafter referred to as "retained γ"). Inorder to prevent embrittlement at the welded parts, thereby obtainingthe best spot weldability, and to obtain an excellent enlargeability(d/d_(o)) of not less than 1.4, the upper limit of C content must beless than 0.15% by weight. When the best enlargeability, d/d_(o) ≧1.5 isneeded, the upper limit must be less than 0.10% by weight. C is also areinforcing element, and the tensile strength will be increased withincreasing C content, but d/d_(o) will be lowered at the same time,rendering the spot weldability inevitably disadvantageous.

Si and Mn are reinforcing elements. Si also promotes formation offerrite (which will be hereinafter referred to as "α"), therebysuppressing formation of carbides. Thus, it has an action to assure theretained γ. Mn has an action to stabilize γ to assure the retained γ. Inorder to fully perform the functions of Si and Mn, it is necessary tocontrol the individual lower limits of Si and Mn and also the lowerlimits of Si+Mn at the same time. That is, it is necessary to controlthe individual lower limits of Si and Mn to not less than 0.5% by weightand the lower limit of Si+Mn to more than 1.5% by weight. Even excessiveaddition of Si and Mn saturates the above-mentioned effects, resultingin deterioration of weldability and slab cracking to the contrary, andthus it is necessary that the individual upper limits of Si and Mn arenot more than 3.0% by weight and the upper limit of Si+Mn is not morethan 6.0% by weight. When a particularly excellent surface state isrequired, it is desirable that a Si content is 1.0 to 2.0% by weight.

P is effective for assuring the retained γ, and in the presentinvention, the upper limit thereof is set to 0.02% by weight to keep thebest secondary workability, toughness and weldability. When therequirements for these characteristics are not so strict, up to 0.2% byweight of P can be added to increase the retained γ.

Upper limit of S is set to 0.01% by weight to prevent deterioration ofenlargeability due to the sulfide-based materials.

Not less than 0.005% by weight of Al is added for deoxidization and toincrease the α volume fraction by making γ grains finer by AIN, make αgrans finer, and increase the retained γ and make the retained γ grainsfiner, and the upper limit is set to 0.10% by weight because ofsaturation of the effects. Up to 3% by weight of Al may be added topromote an increase in the retained γ.

Not less than 0.0005% by weight of Ca is added to control the shape ofsulfide-based materials (spheroidization), and its upper limit is set to0.01% by weight because of saturation of the effects and adverse effectdue to an increase in the sulfide-based materials (deterioration ofenlargeability). For the same reason, an REM content is set to a rangeof 0.005 to 0.05% by weight.

The foregoing are reasons for addition of the main components. At leastone of Nb, Ti, Cr, Cu, Ni, V, B, and Mo may be added in such a range asto assure the strength and make the grains finer, but not as todeteriorate the characteristics.

From the viewpoint of how to obtain the above-mentioned microstructure,values for heating control, rolling control, cooling control, coilingcontrol, etc. and reasons for the control will be explained below.

In order to prevent deterioration of workability due to the appearanceof working structure (working α), particularly the deterioration ofstrength-ductility balance (deterioration of elongation), the lowerlimit of finish-rolling end temperature is set to Ar₃ -50° C. In case ofone-stage cooling (FIG. 6). the upper limit of finish-rolling endtemperature is set to Ar₃ +50° C. to assure the effect on an increase inthe α volume fraction, the effect on making the α grains finer, and theeffect on an increase in the retained γ finer grains in the rollingstep. In case of 2-stage cooling and 3-stage cooling (FIG. 6), as willbe explained later, the effect on an increase in the α volume fraction,the effect on making the α grains finer and the effect on an increase inthe retained γ finer grains can be expected in the cooling step, andthus it is not necessary to set the upper limit of finish-rolling endtemperature, but the upper limit is preferably set to Ar₃ +50° C. tofurther improve the above-mentioned effects.

The entire draft of finish-rolling must be not less than 80% to assurethe effect on an increase in the α volume fraction, the effect on makingthe α grains finer and the effect on an increase in the retained γ finergrains, and preferably the individual draft of 4 passes on the precedingstage must be not less than 40%.

The ultimate pass strain speed of finish-rolling must be not less than30/second to assure the effect on making the α grains finer and theeffect on an increase in the retained γ finer grains.

The lower limit of cooling rate of the one-stage cooling shown in FIG. 6must be 30° C./second to prevent formation of pearlite.

In the two-stage cooling shown in FIG. 6, the first stage cooling mustbe carried out down to not more than Ar₃ at a cooling rate of less than30° C./second to obtain the effect on an increase in the α volumefraction and the effect on an increase in the retained γ finer grains.The second stage cooling must be started from a temperature of more thanAr₁ at a cooling rate of not less than 30° C./second to preventformation of pearlite. It is not objectionable to keep the temperatureconstant in a temperature range of not more than Ar₃ to more than Ar₁.In order to maintain a TRIP phenomenon in a wide range of the strainregion and obtain excellent characteristics, it is desirable to set thefirst stage cooling rate to 5°-20° C./second.

In the three-stage cooling shown in FIG. 6, the first stage cooling mustbe carried out to not more than Ar₃ at a cooling rate of not less than30° C./second to make the α grains finer. The second stage cooling iscarried out at a cooling rate of less than 30° C./second to obtain theeffect on an increase in the α volume fraction and the effect on anincrease in the retained γ finer grains, and the third stage coolingmust be started from more than Ar₁ at a cooling rate of not less than30° C./second to prevent formation of pearlite. It is not objectionableto keep the temperature constant in a range of not more than Ar₃ to morethan Ar₁. In order to maintain a TRIP phenomenon in a wide range ofstrain region and obtain excellent characteristics, it is desirable toset the second stage cooling rate to 5°-20° C./second.

In any of the one-stage cooling, two-stage cooling and three-stagecooling, quenching may be carried out just after the rolling to obtainthe effect on an increase in the α volume fraction, the effect on makingα grains finer and the effect on an increase in the retained γ finergrains or further to reduce the length of the cooling table.

Lower limit of coiling temperature must be more than 350° C. to preventformation of martensite and assure the retained γ. Its upper limit mustbe less than 500° C. to prevent formation of pearlite, suppressexcessive bainite transformation and assure the retained γ.

The foregoing are reasons for control in the present process. In orderto improve the effect on an increase in the α volume fraction, theeffect on making the α grains finer and the effect on an increase in theretained γ finer grains, means such as 1 to set the upper limit of theheating temperature to 1,170° C., 2 to set the finish-rolling initiationtemperature to not more than "rolling end temperature +100° C.", etc.may be carried out alone or in combination. The upper limit of theheating temperature may be set at 1,170° C. to assure the best surfaceproperty.

Furthermore, cooling after the coiling may be spontaneous cooling orforced cooling. In order to suppress excessive bainite transformationand improve the effect on assuring the retained γ grains, cooling may becarried out down to less than 200° C. at a cooling rate of not less than30° C./hour. Cooling may be carried out in combination with theabove-mentioned heating temperature control and finish-rollinginitiation temperature control.

Slabs for use in the rolling may be any of the so called reheated coldslabs, HCR and HDR, or may be slabs prepared by so called continuoussteel casting.

Hot rolled steel sheets obtained according to the present invention maybe used as plates for plating.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing conditions for making retained γ not lessthan 5%.

FIG. 2 is a diagram showing conditions for making retained γ not lessthan 5%.

FIG. 3 is a diagram showing conditions for making retained γ grainshaving grain sizes of not more than 2 μm, not less than 5%.

FIG. 4 is a diagram showing conditions for improving the spotweldability.

FIG. 5 is a diagram showing conditions for improving an enlargementratio.

FIG. 6 is a diagram showing cooling steps at a cooling table.

BEST MODES FOR CARRYING OUT THE INVENTION

Examples are shown below.

Chemical components other than Fe of steel test pieces are shown inTable 2.

Hot rolled steel sheets according to Examples of the present inventionand Comparative Examples are shown in Tables 3 and 4.

                                      TABLE 2                                     __________________________________________________________________________                                   Other                                          Steel                          additive                                       species                                                                           C  Si                                                                              Mn P  S   Al  Ca  REM element                                                                              Si + Mn                                 __________________________________________________________________________    A   0.05                                                                             1.3                                                                             1.5                                                                              0.020                                                                            0.0002                                                                            0.021                                                                             --  --  --     2.8                                     B   0.09                                                                             0.9                                                                             1.9                                                                              0.015                                                                            0.0003                                                                            0.014                                                                             --  --  --     2.8                                     C   0.09                                                                             1.6                                                                             1.7                                                                              0.018                                                                            0.0004                                                                            0.025                                                                             0.0030                                                                            --  --     3.3                                     D   0.05                                                                             2.1                                                                             1.5                                                                              0.015                                                                            0.0001                                                                            0.028                                                                             --  --  --     3.5                                     E   0.09                                                                             2.0                                                                             1.1                                                                              0.010                                                                            0.0002                                                                            0.030                                                                             --  --  --     3.1                                     F   0.09                                                                             0.9                                                                             2.1                                                                              0.008                                                                            0.0003                                                                            0.015                                                                             --  0.010                                                                             --     3.0                                     G   0.08                                                                             1.5                                                                             1.5                                                                              0.015                                                                            0.0002                                                                            0.012                                                                             --  --  Nb = 0.025                                                                           3.0                                     H   0.07                                                                             1.6                                                                             1.6                                                                              0.016                                                                            0.0002                                                                            0.024                                                                             --  --  Cr = 0.2                                                                             3.2                                     I   0.06                                                                             1.7                                                                             1.5                                                                              0.020                                                                            0.0003                                                                            0.015                                                                             --  --  Ti = 0.02                                                                            3.2                                     J   0.07                                                                             1.5                                                                             1.5                                                                              0.010                                                                            0.0002                                                                            0.018                                                                             --  --  B = 0.0005                                                                           3.0                                     K   0.05                                                                             1.4                                                                             1.6                                                                              0.020                                                                            0.0002                                                                            0.014                                                                             --  --  V = 0.03                                                                             3.0                                     L   0.08                                                                             1.8                                                                             1.4                                                                              0.015                                                                            0.0002                                                                            0.013                                                                             --  --  Mo = 0.2                                                                             3.2                                     M   0.10                                                                             1.5                                                                             1.5                                                                              0.018                                                                            0.0002                                                                            0.020                                                                             --  --  --     3.0                                     N   0.14                                                                             1.0                                                                             1.3                                                                              0.015                                                                            0.0002                                                                            0.015                                                                             --  --  --     2.3                                     O   0.10                                                                             2.0                                                                             1.1                                                                              0.001                                                                            0.001                                                                             0.011                                                                             --  --  --     3.1                                     P   0.14                                                                             1.3                                                                             1.3                                                                              0.009                                                                            0.003                                                                             0.024                                                                             --  --  --     2.6                                     Q   0.13                                                                             1.0                                                                             2.0                                                                              0.015                                                                            0.004                                                                             0.020                                                                             --  0.013                                                                             --     3.0                                     R   0.10                                                                             1.5                                                                             1.5                                                                              0.012                                                                            0.002                                                                             0.018                                                                             --  --  V = 0.02                                                                             3.0                                     S   0.11                                                                             1.6                                                                             1.4                                                                              0.018                                                                            0.002                                                                             0.017                                                                             --  --  B = 0.0004                                                                           3.0                                     T   0.10                                                                             2.0                                                                             1.1                                                                              0.019                                                                            0.001                                                                             0.020                                                                             --  --  Ti = 0.01                                                                            3.1                                     U   0.11                                                                             1.8                                                                             1.2                                                                              0.017                                                                            0.002                                                                             0.015                                                                             --  --  Cr = 0.1                                                                             3.0                                     V   0.10                                                                             1.5                                                                             1.5                                                                              0.015                                                                            0.002                                                                             0.015                                                                             --  --  Nb = 0.015                                                                           3.0                                     W   0.10                                                                             1.5                                                                             1.5                                                                              0.017                                                                            0.0004                                                                            0.020                                                                             0.0040                                                                            --  --     3.0                                     X   0.11                                                                             1.7                                                                             1.4                                                                              0.014                                                                            0.002                                                                             0.011                                                                             --  --  Mo = 0.1                                                                             3.1                                     Y   0.05                                                                             1.3                                                                             1.5                                                                              0.018                                                                            0.0001                                                                            0.014                                                                             0.0035                                                                            --  --     2.8                                     Z   0.14                                                                             1.0                                                                             1.3                                                                              0.018                                                                            0.0003                                                                            0.017                                                                             0.0030                                                                            --  --     2.3                                     AA  0.07                                                                             2.0                                                                             2.0                                                                              0.020                                                                            0.002                                                                             0.016                                                                             0.0025                                                                            --  --     4.0                                     AB  0.20                                                                             1.5                                                                             1.5                                                                              0.018                                                                            0.002                                                                             0.015                                                                             0.0030                                                                            --  --     3.0                                     AC  0.13                                                                             0.3                                                                             1.2                                                                              0.017                                                                            0.0002                                                                            0.018                                                                             --  --  --     1.5                                     AA1 0.07                                                                             3.0                                                                             3.0                                                                              0.020                                                                            0.0002                                                                            0.015                                                                             0.0030                                                                            --  --     6.0                                     AA2 0.28                                                                             2.8                                                                             2.8                                                                              0.010                                                                            0.0001                                                                            0.030                                                                             --  --  --     5.6                                     AA3 0.32                                                                             2.8                                                                             2.8                                                                              0.009                                                                            0.0001                                                                            0.010                                                                             --  --  --     5.6                                     __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________                  Microstructure                                                            Steel                                                                             V.sub.F                                                                          d.sub.F                                                                          V.sub.F                                                                          γ.sub.R                                                                    V.sub.B                                                                          V.sub.P                                                                          V.sub.M                                                                          Grain size                                 Distinction                                                                          No.                                                                              species                                                                           (%)                                                                              (μm)                                                                          d.sub.F                                                                          (%)                                                                              (%)                                                                              (%)                                                                              (%)                                                                              of γ.sub.R                           __________________________________________________________________________    The invention                                                                        1  A   88 4.00                                                                             22.0                                                                             5  7  0  0  ≦2 μm                            The invention                                                                        2  B   70 3.24                                                                             21.6                                                                             5  25 0  0  ≦2 μm                            The invention                                                                        3  C   84 3.59                                                                             23.4                                                                             10 6  0  0  ≦2 μm                            The invention                                                                        4  D   84 3.49                                                                             24.1                                                                             9  7  0  0  ≦2 μm                            The invention                                                                        5  E   84 3.59                                                                             23.4                                                                             10 6  0  0  ≦2 μm                            The invention                                                                        6  F   73 3.33                                                                             21.9                                                                             6  21 0  0  ≦2 μm                            The invention                                                                        7  M   69 3.25                                                                             21.2                                                                             5  26 0  0  ≦2 μm                            The invention                                                                        8  N   60 2.99                                                                             20.1                                                                             5  35 0  0  ≦2 μm                            The invention                                                                        9  O   78 3.45                                                                             22.6                                                                             9  13 0  0  ≦2 μ m                           The invention                                                                        10 P   74 3.43                                                                             21.6                                                                             10 16 0  0  ≦2 μm                            The invention                                                                        11 Q   78 3.45                                                                             22.6                                                                             12 10 0  0  ≦2 μm                            The invention                                                                        12 W   78 3.45                                                                             22.6                                                                             9  13 0  0  ≦2 μm                            The invention                                                                        13 Y   80 3.42                                                                             23.4                                                                             7  13 0  0  ≦2 μm                            The invention                                                                        14 Z   63 3.09                                                                             20.4                                                                             6  31 0  0  ≦2 μm                            The invention                                                                        15 AA  78 3.38                                                                             23.1                                                                             8  14 0  0  ≦2 μm                            The invention                                                                        16 AB  56.6                                                                             2.83                                                                             20.0                                                                             5  44 0  0  ≦2 μm                            The invention                                                                        17 AA1 75 3.00                                                                             25.0                                                                             10 15 0  0  ≦2 μm                            The invention                                                                        18 AA2 40 3.00                                                                             13.0                                                                             13 43 0  0  ≦2 μm                            Comp. Ex.                                                                            19 AC  61 2.90                                                                             21.0                                                                             0  39 0  0  --                                         Comp. Ex.                                                                            20 Z   80 3.76                                                                             21.3                                                                             2  11 7  0  ≦2 μm                            Comp. Ex.                                                                            21 B   79 3.46                                                                             22.8                                                                             1  12 0  8  ≦2 μm                            Comp. Ex.                                                                            22 Z   80 3.75                                                                             21.3                                                                             5  15 0  0  >2 μm                                   Comp. Ex.                                                                            23 AA3 24 3.00                                                                             8.0                                                                              13 61 0  0  ≦2 μm                            __________________________________________________________________________

                                      TABLE 4                                     __________________________________________________________________________                  Characteristics of steel sheet                                            Steel                                                                             TS/YP YR T.El/U.El      Spot  Secondary                                                                           Tough-                                                                            Surface                                                                            Bend-              Distinction                                                                          No.                                                                              species                                                                           (kgf/mm.sup.2)                                                                      (%)                                                                              (%)   TS × T.El                                                                     d/d.sub.o                                                                        weldability                                                                         workability                                                                         ness                                                                              state                                                                              ability            __________________________________________________________________________    The invention                                                                        1  A   52/41 78.8                                                                             42.5/27.7                                                                           2210  1.71                                                                             ◯                                                                       ◯                                                                       ◯                                                                     ⊚                                                                   ◯      The invention                                                                        2  B   60/46 76.7                                                                             37.2/24.2                                                                           2230  1.55                                                                             ◯                                                                       ◯                                                                       ◯                                                                     ◯                                                                      ◯      The invention                                                                        3  C   67.5/57                                                                             84.4                                                                             38.8/25.9                                                                           2620  1.58                                                                             ◯                                                                       ◯                                                                       ◯                                                                     ⊚                                                                   ◯      The invention                                                                        4  D   62.5/54                                                                             86.4                                                                             40.5/25.8                                                                           2530  1.68                                                                             ◯                                                                       ◯                                                                       ◯                                                                     ◯                                                                      ◯      The invention                                                                        5  E   64.5/54                                                                             83.7                                                                             40.2/27.3                                                                           2590  1.55                                                                             ◯                                                                       ◯                                                                       ◯                                                                     ⊚                                                                   ◯      The invention                                                                        6  F   63/49 77.8                                                                             36.2/23.6                                                                           2280  1.58                                                                             ◯                                                                       ◯                                                                       ◯                                                                     ◯                                                                      ◯      The invention                                                                        7  M   65/49 75.4                                                                             33.8/20.8                                                                           2200  1.50                                                                             ◯                                                                       ◯                                                                       ◯                                                                     ⊚                                                                   ◯                                                                 3                  The invention                                                                        8  N   83.5/59                                                                             70.7                                                                             26.2/15.4                                                                           2190  1.45                                                                             ◯                                                                       ◯                                                                       ◯                                                                     ⊚                                                                   ◯      The invention                                                                        9  O   66.5/54                                                                             81.2                                                                             37.9/25.0                                                                           2520  1.50                                                                             ◯                                                                       ◯                                                                       ◯                                                                     ⊚                                                                   ◯      The invention                                                                        10 P   67/52 77.6                                                                             38.8/27.7                                                                           2600  1.46                                                                             ◯                                                                       ◯                                                                       ◯                                                                     ⊚                                                                   ◯      The invention                                                                        11 Q   71/58 81.7                                                                             38.9/27.8                                                                           2760  1.48                                                                             ◯                                                                       ◯                                                                       ◯                                                                     ⊚                                                                   ◯      The invention                                                                        12 W   65/53 81.5                                                                             38.6/25.9                                                                           2510  1.53                                                                             ◯                                                                       ◯                                                                       ◯                                                                     ⊚                                                                   ◯      The invention                                                                        13 Y   52/44 84.6                                                                             45.4/30.2                                                                           2360  1.73                                                                             ◯                                                                       ◯                                                                       ◯                                                                     ⊚                                                                   ◯      The invention                                                                        14 Z   67/48 71.6                                                                             34.2/23.3                                                                           2290  1.46                                                                             ◯                                                                       ◯                                                                       ◯                                                                     ⊚                                                                   ◯      The invention                                                                        15 AA  74/61 82.4                                                                             32.8/18.9                                                                           2430  1.62                                                                             ◯                                                                       ◯                                                                       ◯                                                                     ⊚                                                                   ◯      The invention                                                                        16 AB  85/68 80.0                                                                             28.0/18.0                                                                           2380  1.34                                                                             Δ                                                                             ◯                                                                       ◯                                                                     ⊚                                                                   ◯      The invention                                                                        17 AA1 85/60 70.5                                                                             26.0/15.0                                                                           2210  1.42                                                                             ◯                                                                       ◯                                                                       ◯                                                                     ◯                                                                      ◯      The invention                                                                        18 AA2 110/90                                                                              81.8                                                                             22.0/12.0                                                                           2420  1.2                                                                              Δ                                                                             ◯                                                                       ◯                                                                     ◯                                                                      ◯      Comp. Ex.                                                                            19 AC  60/41 74.5                                                                             28.3/14.1                                                                           1700  1.48                                                                             ◯                                                                       ◯                                                                       ◯                                                                     ◯                                                                      ◯      Comp. Ex.                                                                            20 Z   67/50 74.6                                                                             25.4/13.5                                                                           1700  1.38                                                                             ◯                                                                       x     x   ⊚                                                                   x                  Comp. Ex.                                                                            21 B   80/44 55 23.8/14.9                                                                           1900  1.22                                                                             ◯                                                                       x     x   ⊚                                                                   x                  Comp. Ex.                                                                            22 Z   66/49 74.2                                                                             26.5/14.5                                                                           1749  1.29                                                                             ◯                                                                       x     x   ⊚                                                                   x                  Comp. Ex.                                                                            23 AA3 123/100                                                                             81.3                                                                             20.5/12.0                                                                           2251  1.05                                                                             x     ◯                                                                       ◯                                                                     ◯                                                                      ◯      __________________________________________________________________________

Nos. 1 to 18 relate to examples of the present invention, where highyield ratio-type, hot rolled high strength steel sheets excellent inboth formability and spot weldability could be obtained. However, No. 16and No. 18 had a somewhat lower spot weldability due to a higher Ccontent, but had a good workability.

Good surface property was obtained. Particularly good surface propertywas obtained in Nos. 1, 3, 5, and 7 to 16, because the Si content was ina range of 1.0 to 2.0% by weight.

Nos. 19 to 23 relate to Comparative Examples, where No. 19 had lower Sicontent and Si+Mn content than the lower limit, and no retained γ wasobtained and both strength-ductility balance and uniform elongation weredeteriorated; No. 20 contained pearlite and lower retained γ contentthan 5%, and thus the strength-ductility balance, uniform elongation,enlargeability, bendability, secondary workability and toughness weredeteriorated; No. 21 contained martensite and had lower retained γcontent than 5%, and the strength-ductility balance, uniform elongation,enlargeability, bendability, secondary workability and toughness weredeteriorated, and the yield ratio was lower than 60%; No. 22 maintained5% of retained γ content, but its grain size was more than 2 μm, andthus the strength-ductility balance, uniform elongation, enlargeability,bendability, secondary workability and toughness were deteriorated; andNo. 23 had a higher C content than the upper limit and thus the spotweldability and enlargeability were deteriorated.

Even in the steel species G-L, R-V and X of Table 2, high yield ratio,hot rolled high strength steel sheets excellent in both of formabilityand spot weldability could be obtained, and their surface states werealso better.

Processes for producing hot rolled steel sheets according to examples ofthe present invention and comparative examples are shown in Table 5 to10.

                                      TABLE 5                                     __________________________________________________________________________    Examples of one-stage cooling                                                               Production conditions                                                                             Finish-                                                        Finish-    Finish-                                                                           rolling                                                        rolling                                                                            Finish-                                                                             rolling                                                                           ultimate                                                  Heating                                                                            initiation                                                                         rolling                                                                             entire                                                                            pass strain                                                                         Cooling                                                                            Coiling                                                                           Cooling                                Steel                                                                             temp.                                                                              temp.                                                                              end temp.                                                                           draft                                                                             speed/                                                                              rate temp.                                                                             after                        Distinction                                                                          No.                                                                              species                                                                           °C.                                                                         °C.                                                                         °C.                                                                          %   second                                                                              °C./sec                                                                     °C.                                                                        coiling                      __________________________________________________________________________    The invention                                                                        24 C   1170 905  800   93  200   40   360 Spontaneous                  The invention                                                                        25 C   1100 895  790   88  180   35   375 Spontaneous                  The invention                                                                        26 C   1200 860  800   89  40    45   390 Spontaneous                  The invention                                                                        27 C   1050 920  850   92  100   50   380 Spontaneous                  The invention                                                                        28 C   1150 900  810    96*                                                                              300   50   450 Spontaneous                  The invention                                                                        29 C   1180 910  800   94  190    75**                                                                              420 40° C./hr             The invention                                                                        30 AA1 1190 920  810   92  70    50   400 Spontaneous                  Comp. Ex.                                                                            31 C   1180 850  740   95  100   45   505 Spontaneous                  Comp. Ex.                                                                            32 C   1170 900  820   93  20    20   380 Spontaneous                  Comp. Ex.                                                                            33 C   1160 905  810   91  150   50   550 Spontaneous                  Comp. Ex.                                                                            34 C   1200 910  800   89  120   45   300 Spontaneous                  Comp. Ex.                                                                            35 C   1170 920  860   93  20    60   395 Spontaneous                  __________________________________________________________________________     *At least 40% for receding four passes                                        **Quenching right after finishrolling                                    

                                      TABLE 6                                     __________________________________________________________________________    Examples of one-stage cooling                                                 __________________________________________________________________________                  Microstructure                                                                    γ.sub.R                                                                 (grain                                                                    V.sub.F /                                                                         size:        Steel sheet characterisitcs                              Steel                                                                             d.sub.F                                                                           ≦2 μm)                                                                           TS/YP                                                                              YR T.El/U.El                              Distinction                                                                          No.                                                                              species                                                                           ≧ 20                                                                       ≧ 5%                                                                        P   M   kgf/mm.sup.2                                                                       %  %                                      __________________________________________________________________________    The invention                                                                        24 C   ◯                                                                     ◯                                                                      none                                                                              none                                                                              68/57                                                                              83.8                                                                             38.6/25.0                              The invention                                                                        25 C   ◯                                                                     ◯                                                                      none                                                                              none                                                                              67.5/56.5                                                                          83.7                                                                             39.0/26.0                              The invention                                                                        26 C   ◯                                                                     ◯                                                                      none                                                                              none                                                                              67/56                                                                              83.6                                                                             39.2/26.2                              The invention                                                                        27 C   ◯                                                                     ◯                                                                      none                                                                              none                                                                              69/56                                                                              81.2                                                                             37/24                                  The invention                                                                        28 C   ◯                                                                     ◯                                                                      none                                                                              none                                                                              67.3/57                                                                            84.7                                                                             37.5/26.3                              The invention                                                                        29 C   ◯                                                                     ◯                                                                      none                                                                              none                                                                              66.5/56.5                                                                          85.0                                                                             39.6/26.7                              The invention                                                                        30 AA1 ◯                                                                     ◯                                                                      none                                                                              none                                                                              80.2/59.5                                                                          74.2                                                                             32.4/20.5                              Comp. Ex.                                                                            31 C   x*  x    yes none                                                                              65.0/58.0                                                                          89.2                                                                             26.1/14.8                              Comp. Ex.                                                                            32 C   ◯                                                                     x    yes none                                                                              65/54                                                                              83.1                                                                             27.0/14                                Comp. Ex.                                                                            33 C   ◯                                                                     x    yes none                                                                              63/52                                                                              82.5                                                                             27.2/14                                Comp. Ex.                                                                            34 C   ◯                                                                     x    none                                                                              yes 80/43                                                                              51.2                                                                             24.9/14.9                              Comp. Ex.                                                                            35 C   x   x    none                                                                              none                                                                              69.5/48.7                                                                          70.0                                                                             26.5/14.5                              __________________________________________________________________________                  Steel sheet characteristics                                                            Spot                                                                              Sec.                                                         Steel        weld-                                                                             work-                                                                             Tough-                                                                            Surface                                                                            Bend-                                 Distinction                                                                          No.                                                                              species                                                                           TS × T.El                                                                     d/d.sub.o                                                                        ability                                                                           ability                                                                           ness                                                                              state                                                                              ability                               __________________________________________________________________________    The invention                                                                        24 C   2625  1.57                                                                             ◯                                                                     ◯                                                                     ◯                                                                     ⊚                                                                   ◯                         The invention                                                                        25 C   2633  1.58                                                                             ◯                                                                     ◯                                                                     ◯                                                                     ⊚                                                                   ◯                         The invention                                                                        26 C   2626  1.58                                                                             ◯                                                                     ◯                                                                     ◯                                                                     ⊚                                                                   ◯                         The invention                                                                        27 C   2553  1.56                                                                             ◯                                                                     ◯                                                                     ◯                                                                     ⊚                                                                   ◯                         The invention                                                                        28 C   2658  1.58                                                                             ◯                                                                     ◯                                                                     ◯                                                                     ⊚                                                                   ◯                         The invention                                                                        29 C   2633  1.57                                                                             ◯                                                                     ◯                                                                     ◯                                                                     ⊚                                                                   ◯                         The invention                                                                        30 AA1 2598  1.48                                                                             ◯                                                                     ◯                                                                     ◯                                                                     ⊚                                                                   ◯                         Comp. Ex.                                                                            31 C   1697  1.39                                                                             ◯                                                                     x   x   ⊚                                                                   x                                     Comp. Ex.                                                                            32 C   1755  1.39                                                                             ◯                                                                     x   x   ⊚                                                                   x                                     Comp. Ex.                                                                            33 C   1714  1.39                                                                             ◯                                                                     x   x   ⊚                                                                   x                                     Comp. Ex.                                                                            34 C   1992  1.23                                                                             ◯                                                                     x   x   ⊚                                                                   x                                     Comp. Ex.                                                                            35 C   1842  1.50                                                                             ◯                                                                     x   x   ⊚                                                                   ◯                         __________________________________________________________________________     *Working structure (working α) formed                              

                                      TABLE 7                                     __________________________________________________________________________    Examples of two-stage cooling                                                               Production conditions                                                                             Finish-                                                        Finish-    Finish-                                                                           rolling      Cooling                                           rolling                                                                            Finish-                                                                             rolling                                                                           ultimate                                                                            Cooling rate                                                                         rate                                         Heating                                                                            initiation                                                                         rolling                                                                             entire                                                                            pass strain                                                                         CR.sub.1                                                                         CR.sub.2                                                                          shift                                                                              Coiling                                                                           Cooling                         Steel                                                                             temp.                                                                              temp.                                                                              end temp.                                                                           draft                                                                             speed/                                                                              °C./                                                                      °C./                                                                       temp. T.sub.1                                                                      temp.                                                                             after                 Distinction                                                                          No.                                                                              species                                                                           °C.                                                                         °C.                                                                         °C.                                                                          %   sec   sec                                                                              sec °C.                                                                         °C.                                                                        coiling               __________________________________________________________________________    The invention                                                                        36 B   1160 915  810   93  150   15 105 760  400 Spontaneous           The invention                                                                        37 B   1175 900  820   92  190   5  60  780  385 Spontaneous           The invention                                                                        38 B   1150 960  830    94*                                                                              100   9  50  770  415 Spontaneous           The invention                                                                        39 B   1180 940  820   89  180   10 80  760  400 Spontaneous           The invention                                                                        40 B   1200 950  830   91  190   12 60  770  380 35° C./hr      The invention                                                                        41 AA1 1190 945  830   91  210   12 60  770  390 Spontaneous           Comp. Ex.                                                                            42 B   1100 800  720   92  150   13 75  680  510 Spontaneous           Comp. Ex.                                                                            43 B   1190 930  840   77  100   25 80  750  450 Spontaneous           Comp. Ex.                                                                            44 B   1180 990  870   91  190   40 85  650  440 Spontaneous           Comp. Ex.                                                                            45 B   1170 950  840   90  120   25 20  700  500 Spontaneous           Comp. Ex.                                                                            46 B   1160 945  830   93  20    19 90  590  480 Spontaneous           Comp. Ex.                                                                            47 B   1200 970  860   89  50    10 45  820  400 Spontaneous           __________________________________________________________________________     *At least 40% for preceding four passes                                  

                                      TABLE 8                                     __________________________________________________________________________    Examples of two-stage cooling                                                 __________________________________________________________________________                  Microstructure                                                                    γ.sub.R                                                                 (grain                                                                    V.sub.F /                                                                         size:        Steel sheet characterisitcs                              Steel                                                                             d.sub.F                                                                           ≦2 μm)                                                                           TS/YP                                                                              YR T.El/U.El                              Distinction                                                                          No.                                                                              species                                                                           ≧ 20                                                                       ≧ 5%                                                                        P   M   kgf/mm.sup.2                                                                       %  %                                      __________________________________________________________________________    The invention                                                                        36 B   ◯                                                                     ◯                                                                      none                                                                              none                                                                              60/47                                                                              78.3                                                                             37.1/24.2                              The invention                                                                        37 B   ◯                                                                     ◯                                                                      none                                                                              none                                                                              59/47                                                                              79.7                                                                             38.0/25.0                              The invention                                                                        38 B   ◯                                                                     ◯                                                                      none                                                                              none                                                                              60/46                                                                              76.7                                                                             38.5/26                                The invention                                                                        39 B   ◯                                                                     ◯                                                                      none                                                                              none                                                                              60.5/47                                                                            77.7                                                                             37.0/24.1                              The invention                                                                        40 B   ◯                                                                     ◯                                                                      none                                                                              none                                                                              60.5/47                                                                            77.7                                                                             38.2/25.8                              The invention                                                                        41 AA1 ◯                                                                     ◯                                                                      none                                                                              none                                                                              81.3/58.2                                                                          71.6                                                                             28.4/18.5                              Comp. Ex.                                                                            42 B   x*  x    yes none                                                                              57/48                                                                              84.2                                                                             27.5/14.8                              Comp. Ex.                                                                            43 B   x   x    none                                                                              none                                                                              62/43.4                                                                            70.0                                                                             28/14                                  Comp. Ex.                                                                            44 B   x   x    none                                                                              none                                                                              65/56.6                                                                            70.0                                                                             27/13                                  Comp. Ex.                                                                            45 B   ◯                                                                     x    yes none                                                                              55/45                                                                              81.8                                                                             28/14.7                                Comp. Ex.                                                                            46 B   ◯                                                                     x    yes none                                                                              56/45                                                                              80.4                                                                             27/14                                  Comp. Ex.                                                                            47 B   x   x    none                                                                              none                                                                              66/46.2                                                                            70.0                                                                             26/13                                  __________________________________________________________________________                  Steel sheet characteristics                                                            Spot                                                                              Sec.                                                         Steel        weld-                                                                             work-                                                                             Tough-                                                                            Surface                                                                            Bend-                                 Distinction                                                                          No.                                                                              species                                                                           TS × T.El                                                                     d/d.sub.o                                                                        ability                                                                           ability                                                                           ness                                                                              state                                                                              ability                               __________________________________________________________________________    The invention                                                                        36 B   2226  1.55                                                                             ◯                                                                     ◯                                                                     ◯                                                                     ⊚                                                                   ◯                         The invention                                                                        37 B   2242  1.56                                                                             ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                      ◯                         The invention                                                                        38 B   2310  1.56                                                                             ◯                                                                     ◯                                                                     ◯                                                                     ⊚                                                                   ◯                         The invention                                                                        39 B   2239  1.55                                                                             ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                      ◯                         The invention                                                                        40 B   2311  1.55                                                                             ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                      ◯                         The invention                                                                        41 AA1 2310  1.43                                                                             ◯                                                                     ◯                                                                     ◯                                                                     ⊚                                                                   ◯                         Comp. Ex.                                                                            42 B   1568  1.39                                                                             ◯                                                                     x   x   ⊚                                                                   x                                     Comp. Ex.                                                                            43 B   1736  1.50                                                                             ◯                                                                     x   x   ◯                                                                      ◯                         Comp. Ex.                                                                            44 B   1755  1.51                                                                             ◯                                                                     x   x   ◯                                                                      ◯                         Comp. Ex.                                                                            45 B   1540  1.38                                                                             ◯                                                                     x   x   ⊚                                                                   x                                     Comp. Ex.                                                                            46 B   1512  1.39                                                                             ◯                                                                     x   x   ⊚                                                                   x                                     Comp. Ex.                                                                            47 B   1716  1.52                                                                             ◯                                                                     x   x   ◯                                                                      ◯                         __________________________________________________________________________     *Working structure (working α) formed                              

                                      TABLE 9                                     __________________________________________________________________________    Examples of three-stage cooling                                                             Production conditions                                                                          Finish-                                                          Finish-                                                                            Finish-                                                                           Finish-                                                                           rolling        Cooling                                       Heat-                                                                             rolling                                                                            rolling                                                                           rolling                                                                           ultimate                                                                            Cooling rate                                                                           rate                                          ing initiation                                                                         end entire                                                                            pass strain                                                                         CR.sub.1                                                                         CR.sub.2                                                                         CR.sub.3                                                                         shift temp                                                                          Coiling                                                                           Cooling                         Steel                                                                             temp.                                                                             temp.                                                                              temp.                                                                             draft                                                                             speed/                                                                              °C./                                                                      °C./                                                                      °C./                                                                      T.sub.1                                                                          T.sub.2                                                                          temp.                                                                             after                 Distinction                                                                          No.                                                                              species                                                                           °C.                                                                        °C.                                                                         °C.                                                                        %   sec   sec                                                                              sec                                                                              sec                                                                              °C.                                                                       °C.                                                                       °C.                                                                        coiling               __________________________________________________________________________    The invention                                                                        48 AA  1170                                                                              900  800  94*                                                                              100   50 5  50 750                                                                              725                                                                              380 40° C./hr      The invention                                                                        49 AA  1190                                                                              970  850 93  50    90 15 90 700                                                                              600                                                                              410 Spontaneous           The invention                                                                        50 C   1200                                                                              930  820 92  80    40 7  40 700                                                                              680                                                                              405 Spontaneous           The invention                                                                        51 C   1180                                                                              960  870 91  190   85 18 85 710                                                                              610                                                                              390 Spontaneous           The invention                                                                        52 C   1190                                                                              970  860 92  210   95 8  100                                                                              650                                                                              600                                                                              390 Spontaneous           The invention                                                                        53 AA1 1185                                                                              960  840 93  150   90 15 90 700                                                                              600                                                                              410 Spontaneous           Comp. Ex.                                                                            54 C   1200                                                                              980  865 94  200   60 35 60 670                                                                              600                                                                              440 Spontaneous           Comp. Ex.                                                                            55 C   1160                                                                              980  870 93  170   80 9  20 660                                                                              600                                                                              480 Spontaneous           Comp. Ex.                                                                            56 C   1200                                                                              990  880 92  180   40 7  60 840                                                                              805                                                                              430 Spontaneous           Comp. Ex.                                                                            57 C   1180                                                                              970  870 82  25    25 15 85 710                                                                              620                                                                              400 Spontaneous           __________________________________________________________________________     *At least 40% for preceding four passes                                  

                                      TABLE 10                                    __________________________________________________________________________    Examples of three-stage cooling                                               __________________________________________________________________________                  Microstructure                                                                    γ.sub.R                                                                 (grain                                                                    V.sub.F /                                                                         size:        Steel sheet characterisitcs                              Steel                                                                             d.sub.F                                                                           ≦2 μm)                                                                           TS/YP                                                                              YR T.El/U.El                              Distinction                                                                          No.                                                                              species                                                                           ≧ 20                                                                       ≧ 5%                                                                        P   M   kgf/mm.sup.2                                                                       %  %                                      __________________________________________________________________________    The invention                                                                        48 AA  ◯                                                                     ◯                                                                      none                                                                              none                                                                              74.2/61                                                                            82.2                                                                             34.8/21.8                              The invention                                                                        49 AA  ◯                                                                     ◯                                                                      none                                                                              none                                                                              73/60.5                                                                            82.9                                                                             34.5/24.5                              The invention                                                                        50 C   ◯                                                                     ◯                                                                      none                                                                              none                                                                              67/57                                                                              85.1                                                                             39/26                                  The invention                                                                        51 C   ◯                                                                     ◯                                                                      none                                                                              none                                                                              68/58                                                                              85.3                                                                             37/24                                  The invention                                                                        52 C   ◯                                                                     ◯                                                                      none                                                                              none                                                                              67/56                                                                              83.6                                                                             38/25                                  The invention                                                                        53 AA1 ◯                                                                     ◯                                                                      none                                                                              none                                                                              85/61                                                                              71.8                                                                             26.2/15.1                              Comp. Ex.                                                                            54 C   x   x    none                                                                              none                                                                              71/49.7                                                                            70.0                                                                             25/12                                  Comp. Ex.                                                                            55 C   ◯                                                                     x    yes none                                                                              64/53                                                                              82.8                                                                             27/14                                  Comp. Ex.                                                                            56 C   x   x    none                                                                              none                                                                              70/49                                                                              70.0                                                                             26/13                                  Comp. Ex.                                                                            57 C   x   x    none                                                                              none                                                                              66/55                                                                              83.3                                                                             27/13                                  __________________________________________________________________________                   Steel sheet characteristics                                                           Spot                                                                              Sec.                                                         Steel        weld-                                                                             work-                                                                             Tough-                                                                            Surface                                                                            Bend-                                 Distinction                                                                          No.                                                                              species                                                                           TS × T.El                                                                     d/d.sub.o                                                                        ability                                                                           ability                                                                           ness                                                                              state                                                                              ability                               __________________________________________________________________________    The invention                                                                        48 AA  2582  1.63                                                                             ◯                                                                     ◯                                                                     ◯                                                                     ⊚                                                                   ◯                         The invention                                                                        49 AA  2519  1.64                                                                             ◯                                                                     ◯                                                                     ◯                                                                     ⊚                                                                   ◯                         The invention                                                                        50 C   2613  1.58                                                                             ◯                                                                     ◯                                                                     ◯                                                                     ⊚                                                                   ◯                         The invention                                                                        51 C   2516  1.59                                                                             ◯                                                                     ◯                                                                     ◯                                                                     ⊚                                                                   ◯                         The invention                                                                        52 C   2546  1.59                                                                             ◯                                                                     ◯                                                                     ◯                                                                     ⊚                                                                   ◯                         The invention                                                                        53 AA1 2227  1.43                                                                             ◯                                                                     ◯                                                                     ◯                                                                     ⊚                                                                   ◯                         Comp. Ex.                                                                            54 C   1775  1.58                                                                             ◯                                                                     x   x   ⊚                                                                   ◯                         Comp. Ex.                                                                            55 C   1728  1.39                                                                             ◯                                                                     x   x   ⊚                                                                   x                                     Comp. Ex.                                                                            56 C   1820  1.59                                                                             ◯                                                                     x   x   ⊚                                                                   ◯                         Comp. Ex.                                                                            57 C   1792  1.50                                                                             ◯                                                                     x   x   ⊚                                                                   ◯                         __________________________________________________________________________

Tables 5 and 6 show processes for producing a hot rolled steel sheet incase of one-stage cooling at the cooling table according to the presentexamples and comparative examples, shown in FIG. 6.

Nos. 24 to 30 relate to examples of the present invention, where highyield ratio-type, hot rolled high strength steel sheets excellent inboth formarbility and spot weldability could be obtained and theirsurface states were found to be better.

Nos. 31 to 35 relate to comparative examples, where No. 31 had a lowerrolling end temperature than the lower limit and a higher coilingtemperature than the upper limit, and thus a working structure (workingα) and pearlite were formed, and not less than 5% by weight of retainedγ having grain sizes of not more than 2 μm could not be obtained, and,as a result, the strength-ductility balance, uniform elongation,enlargeability, bendability, secondary workability and toughness weredeteriorated; No. 32 had a lower finish-rolling ultimate pass strainspeed than the lower limit and a lower cooling rate than the lowerlimit, resulting in formation of pearlite, and not less than 5% ofretained γ having grain sizes of not more than 2 μm could not beobtained, and, as a result, the strength-ductility balance, uniformelongation, enlargeability, bendability, secondary workability andtoughness were deteriorated; No. 33 had a higher coiling temperaturethan the upper limit, resulting in formation of pearlite, and not lessthan 5% of retained γ having grain sizes of not more than 2 μm could notbe obtained, and, as a result, the strength-ductility balance, uniformelongation, enlargeability, bendability, secondary workability andtoughness were deteriorated; No. 34 had a lower coiling temperature thanthe lower limit, resulting in formation of martensite, and not less than5% of retained γ having grain sizes of not more than 2 μm could not beobtained, and, as a result, the strength-ductility balance, uniformelongation, enlargeability, bendability, secondary workability andtoughness were deteriorated, and the yield ratio was lower than 60%; andNo. 35 had a higher finish-rolling end temperature than the upper limitand a lower finish-rolling ultimate pass strain speed than the lowerlimit, resulting in failure to attain such a relationship as V_(F)/d_(F) ≧20, and not less than 5% of retained γ having grain sizes of notmore than 2 μm could not be obtained, and, as a result, thestrength-ductility balance, uniform elongation, secondary workabilityand toughness were deteriorated.

Tables 7 and 8 show processes for producing hot rolled steel sheets incase of two-stage cooling at the cooling table according to the presentexamples and comparative examples, as shown in FIG. 6.

Nos. 36 to 41 relate to examples of the present invention, where highyield ratio-type, hot rolled high strength steel sheets excellent inboth formability and spot weldability could be obtained and theirsurface states were found to be better.

Nos. 42 to 47 relate to comparative examples, where No. 42 had a lowerfinish-rolling end temperature than the lower limit and a higher coilingtemperature than the upper limit, resulting in formation of workingstructure (working α) and pearlite, and not less than 5% of retained γhaving grain sizes of not more than 2 μm could not be obtained, and, asa result, the strength-ductility balance, uniform elongation,enlargeability, bendability, secondary workability and toughness weredeteriorated; No. 43 had a lower entire draft of finish-rolling than thelower limit, resulting in failure to attain such a relation as V_(F)/d_(F) >20, and not more than 5% of retained γ having grain sizes of notless than 2 μm could not be obtained, and, as a result, thestrength-ductility balance, uniform elongation, secondary workabilityand toughness were deteriorated; No. 44 had a higher cooling rate at thefirst stage than the upper limit, resulting in failure to attain such arelation as V_(F) /d_(F) ≧20, and not less than 5% of retained γ havinggrain sizes of not more than 2 μm could not be obtained, and, as aresult, the strength-ductility balance, uniform elongation, secondaryworkability and toughness were deteriorated; No. 45 had a lower coolingrate at the second stage than the lower limit, resulting in formation ofpearlite, and not more than 5% of retained γ having grain sizes of notmore than 2 μm could not be obtained, and, as a result, thestrength-ductility balance, uniform elongation, enlargeability,bendability, secondary workability and toughness were deteriorated; No.46 had a lower finish-rolling ultimate pass strain speed than the lowerlimit and a higher coiling temperature than the upper limit, resultingin formation of pearlite, and not less than 5% of retained γ havinggrain sizes of not more than 2 μm could not be obtained, and, as aresult, the strength-ductility balance, uniform elongation,enlargeability, bendability, secondary workability and toughness weredeteriorated; and No. 47 had a higher cooling end temperature (coolingrate shift temperature T₁) at the first stage than the upper limit,resulting in failure to attain such a relation as V_(F) /d_(F) >20 andnot less than 5% of retained γ having grain size of not more than 2 μmcould not be obtained, and, as a result, the strength-ductility balance,uniform elongation, secondary workability and toughness weredeteriorated.

Tables 9 and 10 show processes for producing hot rolled steel sheets incase of three-stage cooling at the cooling table according to thepresent examples and comparative examples, shown in FIG. 6.

Nos. 48 to 53 relate to examples of the present invention, where highyield ratio-type, hot rolled high strength steel sheets excellent inboth formability and spot weldability could be obtained and theirsurface states were found to be better.

Nos. 54 to 56 relate to comparative examples, where No. 54 had a highercooling rate at the second stage than the upper limit, resulting infailure to attain such a relation as V_(F) /d_(F) ≧20 and not less than5% of retained γ having grain sizes of not more than 2 μm could not beobtained, and, as a result, the strength-ductility balance, uniformelongation, secondary workability and toughness were deteriorated; No.55 had a lower cooling rate at the third stage than the lower limit,resulting in the formation of pearlite, and not less than 5% of retainedγ having grain sizes of not more than 2 μm could not be obtained, and,as a result, the strength-ductility balance, uniform elongation,enlargeability, bendability, secondary workability and toughness weredeteriorated; No. 56 had higher cooling end temperatures (cooling rateshift temperatures T₁ and T₂) at the first and second stages,respectively, than the upper limits, resulting in failure to attain sucha relationship as V_(F) /d_(F) ≧20, and not less than 5% of retained γhaving grain sizes of not more than 2 μm could not be obtained, and, asa result, the strength-ductility balance, uniform elongation, secondaryworkability and toughness were deteriorated; No. 57 had a lowerfinish-rolling ultimate strain speed than the lower limit, resulting infailure to attain such a relation as V_(F) /d_(F) ≧20, and not more than5% of retained γ having grain sizes of not less than 2 μm could not beobtained, and, as a result, the strength-ductility balance, uniformelongation, secondary workability and toughness were deteriorated.

Even in the steel species G-L, R-V and X of Table 2, high yieldratio-type, hot rolled high strength steel sheets having excellentformability and spot weldability together and a good surface state couldbe obtained according to the same processes of the present invention.

As is apparent from the foregoing, various practical cases and parts canbe made available only according to the present invention with combinedcharacteristics.

Evaluation of the characteristics has been made according to thefollowing procedures:

Tensile tests were carried out according to JIS No. 5 to determinetensile strength (TS), yield strength (YP), yield ratio (YR=100×YP/TS),total elongation (T.E1), uniform elongation (U.E1), andstrength-ductility balance (TS×T.E1).

Enlargeability or hole expansibility was expressed by an enlargementratio (d/d_(o)), determined by enlarging a punch hole, 20 mm in diameter(initial diameter:d_(o)), with a 30° cone punch from the flash-free sideto measure a hole diameter (d) when a crack passed through the testpiece in the thickness direction, and obtaining the ratio (d/d_(o)).

Bendability was determined by bending a test piece, 35 mm×70 mm, at a90° V bending angle with 0.5 R at the tip end (bending axis being in therolling direction), while making the flash existing side outside, andnon-occurrence of cracks, 1 mm or longer, was expressed by a round mark"⊚", and the occurrence of such cracks by a crossed mark "X".

Secondary workability was determined by crushing a cup which was shapedfrom a punched plate (punch hole: 90 mm in diameter) at a drawing ratioof 1.8, at -50° C. and non-occurrence of cracks was expressed by a roundmark "⊚" and the occurrence of cracks by a corssed mark "X".

Toughness was expressed by a round mark "⊚" when the test piece wassatisfactory at a transition temperature of -120° C. or less, and by acrossed mark "X" when not.

Spot weldability was determined by dividing a spot-welding test pieceinto two orignial pieces by a chisel and non-occurrence of breakageinside the nugget (portion melted at the spot welding and solidifiedthereafter) was expressed by a round mark "⊚" and the occurrence thereofby a crossed mark "X".

Surface state was visually inspected, and a very good surface state wasexpressed by a double round mark "⊚" and a good surface state by a roundmark "⊚".

Industrial Applicability

In the present invention, a hot rolled high strength steel sheet havingcombined characteristics not found in the prior art, that is, a hotrolled high strength steel sheet having an excellent formability, a highyield ratio and an excellent spot weldability, can be stably produced ata low cost, and applications and service conditions can be considerablyexpanded.

We claim:
 1. A process for producing a high yield ratio-type, hot rolledhigh strength steel sheet having both an excellent formability and spotweldability, and also having a yield ratio (YR) of not less than 60%, astrength-ductility balance (tensile strength×total elongation) of notless than 2,000 (kgf/mm².%), an enlargement ratio (d/d_(o)) of not lessthan 1.4 and a uniform elongation of not less than 15%, which comprisesconducting a finish-rolling of a slab prepared by casting a steelconsisting essentially of 0.05 to less than 0.15% by weight of C, 0.5 to3.0% by weight of Si, 0.5 to 3.0% by weight of Mn, more than 1.5 to 6.0%by weight of Si and Mn in total, not more than 0.02% by weight of P, notmore than 0.01% by weight of S, and 0.005 to 0.10% by weight of Al, thebalance consisting essentially of Fe, being composed of three phases offerrite, bainite and retained austenite as microstructure, and having aferrite grain size (d_(F)) of not more than 5 μm, a ratio (V_(F) /d_(F))of ferrite volume fraction (V_(F)) to ferrite grain size (d_(F)) of notless than 20, and a volume fraction of retained austenite having a grainsize of not more than 2 μm being not less than 5%, at an end temperaturerange of Ar₃ ±50° C. at an entire draft of not less than 80% and anultimate pass strain speed of not less than 30/second, conductingcooling at a hot run table at a rate of not less than 30° C./second, andconducting coiling at a temperature of more than 350° C. to 500° C.
 2. Aprocess for producing a high yield ratio, hot rolled high strength steelsheet having both an excellent formability and spot weldability, andalso having a yield ratio (YR) of not less than 60%, astrength-ductility balance (tensile strength×total elongation) of notless than 2,000 (kgf/mm².%), an enlargement ratio (d/d_(o)) of not lessthan 1.4 and a uniform elongation of not less than 15%, which comprisesconducting a finish-rolling of a slab prepared by casting a steelconsisting essentially of 0.05 to less than 0.15% by weight of C, 0.5 to3.0% by weight of Si, 0.5 to 3.0% by weight of Mn, more than 1.5 to 6.0%by weight of Si and Mn in total, not more than 0.02% by weight of P, notmore than 0.01% by weight of S, and 0.005 to 0.10% by weight of Al, and0.0005 to 0.01% by weight of Ca or 0.005 to 0.05% by weight of REM, thebalance being Fe and inevitable elements, being composed of three phasesof ferrite, bainite and retained austenite as microstructure, and havinga ferrite grain size (d_(F)) of not more than 5 μm, a ratio (V_(F)/d_(F)) of ferrite volume fraction (V_(F)) to ferrite grain size (d_(F))of not less than 20, and a volume fraction of retained austenite havinga grain size of not more than 2 μm being not less than 5%, at an endtemperature range of Ar₃ ±50° C., at an entire draft of not less than80% and an ultimate pass strain speed of not less than 30/second,conducting cooling at a hot run table at a rate of not less than 30°C./second, and conducting coiling at a temperature of more than 350° C.to 500° C.
 3. A process for producing a high yield ratio, hot rolledhigh strength steel sheet having both an excellent formability and spotweldability, and also having a yield ratio (YR) of not less than 60%, astrength-ductility balance (tensile strength×total elongation) of notless than 2,000 (kgf/mm².%), an enlargement ratio (d/d_(o)) of not lessthan 1.4 and a uniform elongation of not less than 15%, which comprisesconducting a finish-rolling of a slab prepared by casting a steelconsisting essentially of 0.05 to less than 0.15% by weight of C, 0.5 to3.0% by weight of Si, 0.5 to 3.0% by weight of Mn, more than 1.5 to 6.0%by weight of Si and Mn in total, not more than 0.02% by weight of P, notmore than 0.01% by weight of S, and 0.005 to 0.10% by weight of Al, thebalance consisting essentially of Fe, being composed of three phases offerrite, bainite and retained austenite as microstructure, and having aferrite grain size (d_(F)) of not more than 5 μm, a ratio (V_(F) /d_(F))of ferrite volume fraction (V_(F)) to ferrite grain size (d_(F)) of notless than 20, and a volume fraction of retained austenite having a grainsize of not more than 2 μm being not less than 5%, at an end temperatureof not less than Ar₃ -50° C., at an entire draft of not less than 80%and an ultimate pass strain speed of not less than 30/second, conductingcooling at a hot run table down to a temperature T₁ in a range of notmore than Ar₃ to more than Ar₁ at a rate of less than 30° C./second, andfrom T₁ downwards at a rate of not less than 30° C./second, andconducting coiling at a temperature of more than 350° C. to 500° C.
 4. Aprocess for producing a high yield ratio, hot rolled high strength steelsheet having both an excellent formability and spot weldability, andalso having a yield ratio (YR) of not less than 60%, astrength-ductility balance (tensile strength×total elongation) of notless than 2,000 (kgf/mm².%), an enlargement ratio (d/d_(o)) of not lessthan 1.4 and a uniform elongation of not less than 15%, which comprisesconducting a finish-rolling of a slab prepared by casting a steelconsisting essentially of 0.05 to less than 0.15% by weight of C, 0.5 to3.0% by weight of Si, 0.5 to 3.0% by weight of Mn, more than 1.5 to 6.0%by weight of Si and Mn in total, not more than 0.02% by weight of P, notmore than 0.01% by weight of S, and 0.005 to 0.10% by weight of Al, and0.0005 to 0.01% by weight of Ca or 0.005 to 0.05% by weight of REM, thebalance being Fe and inevitable elements, being composed of three phasesof ferrite, bainite and retained austenite as microstructure, and havinga ferrite grain size (d_(F)) of not more than 5 μm, a ratio (V_(F)/d_(F)) of ferrite volume fraction (V_(F)) to ferrite grain size (d_(F))of not less than 20, and a volume fraction of retained austenite havinga grain size of not more than 2 μm being not less than 5%, at an endtemperature of not less than Ar₃ -50° C. at an entire draft of not lessthan 80% and an ultimate pass strain speed of not less than 30/second,conducting cooling at a hot run table down to a temperature T₁ in arange of not more than Ar₃ to more than Ar₁, at a rate of less than 30°C./second, and from T₁ downwards at a rate of not less than 30°C./second, and conducting coiling at a temperature of more than 350° C.to 500° C.
 5. A process for producing a high yield ratio, hot rolledhigh strength steel sheet having both an excellent formability and spotweldability, and also having a yield ratio (YR) of not less than 60%, astrength-ductility balance (tensile strength×total elongation) of notless than 2,000 (kgf/mm².%), an enlargement ratio (d/d_(o)) of not lessthan 1.4 and a uniform elongation of not less than 15%, which comprisesconducting a finish-rolling of a slab prepared by casting a steelconsisting essentially of 0.05 to less than 0.15% by weight of C, 0.5 to3.0% by weight of Si, 0.5 to 3.0% by weight of Mn, more than 1.5 to 6.0%by weight of Si and Mn in total, not more than 0.02% by weight of P, notmore than 0.01% by weight of S, and 0.005 to 0.10% by weight of Al, thebalance consisting essentially of Fe, being composed of three phases offerrite, bainite and retained austenite as microstructure, and having aferrite grain size (d_(F)) of not more than 5 μm, a ratio (V_(F) /d_(F))of ferrite volume fraction (V_(F)) to ferrite grain size (d_(F)) of notless than 20, and a volume fraction of retained austenite having a grainsize of not more than 2 μm being not less than 5%, at an end temperatureof not less than Ar₃ -50° C., at an entire draft of not less than 80%and an ultimate pass strain speed of not less than 30/second, conductingcooling at a hot run table down to a temperature T₁ in a range of notmore than Ar₃ to more than Ar₁ at a rate of not less than 30° C./second,and from T₁ downwards to a temperature T₂ in a range of not more than T₁to more than Ar₁ at a rate of less than 30° C./second, and furthermorefrom T₂ downwards at a rate of not less than 30° C./second, andconducting coiling at a temperature of more than 350° C. to 500° C.
 6. Aprocess for producing a high yield ratio, hot rolled high strength steelsheet having both an excellent formability and spot weldability, andalso having a yield ratio (YR) of not less than 60%, astrength-ductility balance (tensile strength×total elongation) of notless than 2,000 (kgf/mm².%), an enlargement ratio (d/d_(o)) of not lessthan 1.4 and a uniform elongation of not less than 15%, which comprisesconducting a finish-rolling of a slab prepared by casting a steelconsisting essentially of 0.05 to less than 0.15% by weight of C, 0.5 to3.0% by weight of Si, 0.5 to 3.0% by weight of Mn, more than 1.5 to 6.0%by weight of Si and Mn in total, not more than 0.02% by weight of P, notmore than 0.01% by weight of S, and 0.005 to 0.10% by weight of Al, and0.0005 to 0.01% by weight of Ca or 0.005 to 0.05% by weight of REM, thebalance being Fe and inevitable elements, being composed of three phasesof ferrite, bainite and retained austenite as microstructure, and havinga ferrite grain size (d_(F)) of not more than 5 μm, a ratio (V_(F)/d_(F)) of ferrite volume fraction (V_(F)) to ferrite grain size (d_(F))of not less than 20, and a volume fraction of retained austenite havinga grain size of not more than 2 μm being not less than 5%, at an endtemperature of not less than Ar₃ -50° C. at an entire draft of not lessthan 80% and an ultimate pass strain speed of not less than 30/second,conducting cooling at a hot run table down to a temperature T₁ in arange of not more than Ar₃ to more than Ar₁ at a rate of not less than30° C./second, and from T₁ downwards to a temperature T₂ in a range ofnot more than T₁ to more than Ar₁ at a rate of less than 30° C./second,and furthermore from T₂ downwards at a rate of not less than 30°C./second, and conducting coiling at a temperature of more than 350° C.to 500° C.
 7. A process for producing a high yield ratio, hot rolledhigh strength steel sheet having an excellent formability and also ayield ratio (YR) of not less than 60%, a strength-ductility balance(tensile strength×total elongation) of not less than 2,000 (kgf/mm².%),an enlargement ratio (d/d_(o)) of not less than 1.1, and a uniformelongation of not less than 10%, which comprises conducting afinish-rolling of a slab prepared by casting a steel consistingessentially of 0.15 to less than 0.30% by weight of C, 2.0 to 3.0% byweight of Si, 0.5 to 3.0% by weight of Mn, more than 2.5 to 6.0% byweight of Si and Mn in total, not more than 0.02% by weight of P, notmore than 0.01% by weight of S, and 0.005 to 0.10% by weight of Al, thebalance consisting essentially of Fe, being composed of three phases offerrite, bainite and retained austenite as microstructure, and having aferrite grain size (d_(F)) of not more than 5 μm, a ratio (V_(F) /d_(F))of ferrite volume fraction (V_(F)) to ferrite grain size (d_(F)) of notless than 7, and a volume fraction of retained austenite having a grainsize of not more than 2 μm being not less than 5%, at an end temperaturerange of Ar₃ ±50° C. at an entire draft of not less than 80% and anultimate pass strain speed of not less than 30/second, conductingcooling at a hot run table at a rate of not less than 30° C./second, andconducting coiling at a temperature of more than 350° C. to 500° C.
 8. Aprocess for producing a high yield ratio, hot rolled high strength steelsheet having an excellent formability and also a yield ratio (YR) of notless than 60%, a strength-ductility balance (tensile strength×totalelongation) of not less than 2,000 (kgf/mm².%), an enlargement ratio(d/d_(o)) of not less than 1.1, and a uniform elongation of not lessthan 10%, which comprises conducting a finish-rolling of a slab preparedby casting a steel consisting essentially of 0.15 to less than 0.30% byweight of C, 2.0 to 3.0% by weight of Si, 0.5 to 3.0% by weight of Mn,more than 2.5 to 6.0% by weight of Si and Mn in total, not more than0.02% by weight of P, not more than 0.01% by weight of S, and 0.005 to0.10% by weight of Al, and 0.0005 to 0.01% by weight of Ca or 0.005 to0.05% by weight of REM, the balance being Fe and inevitable elements,being composed of three phases of ferrite, bainite and retainedaustenite as microstructure, and having a ferrite grain size (d_(F)) ofnot more than 5 μm, a ratio (V_(F) /d_(F)) of ferrite volume fraction(V_(F)) to ferrite grain size (d_(F)) of not less than 7, and a volumefraction of retained austenite having a grain size of not more than 2 μmbeing not less than 5%, at an end temperature range of Ar₃ ±50° C. at anentire draft of not less than 80% and an ultimate pass strain speed ofnot less than 30/second, conducting cooling at a hot run table at a rateof not less than 30° C./second, and conducting coiling at a temperatureof more than 350° C. to 500° C.
 9. A process for producing a high yieldratio, hot rolled high strength steel sheet having an excellentformability and also a yield ratio (YR) of not less than 60%, astrength-ductility balance (tensile strength×total elongation) of notless than 2,000 (kgf/mm².%), an enlargement ratio (d/d_(o)) of not lessthan 1.1, and a uniform elongation of not less than 10%, which comprisesconducting a finish-rolling of a slab prepared by casting a steelconsisting essentially of 0.15 to less than 0.30% by weight of C, 2.0 to3.0% by weight of Si, 0.5 to 3.0% by weight of Mn, more than 2.5 to 6.0%by weight of Si and Mn in total, not more than 0.02% by weight of P, notmore than 0.01% by weight of S, and 0.005 to 0.10% by weight of Al, thebalance consisting essentially of Fe, being composed of three phases offerrite, bainite and retained austenite as microstructure, and having aferrite grain size (d_(F)) of not more than 5 μm, a ratio (V_(F) /d_(F))of ferrite volume fraction (V_(F)) to ferrite grain size (d_(F)) of notless than 7, and a volume fraction of retained austenite having a grainsize of not more than 2 μm being not less than 5%, at an end temperatureof not less than Ar₃ -50° C., at an entire draft of not less than 80%and an ultimate pass strain speed of not less than 30/second, conductingcooling at a hot run table down to a temperature T₁ in a range of notmore than Ar₃ to more than Ar₁ at a rate of less than 30° C./second, andfrom T₁ downwards at a rate of not less than 30° C./second, andconducting coiling at a temperature of more than 350° C. to 500° C. 10.A process for producing a high yield ratio, hot rolled high strengthsteel sheet having an excellent formability and also a yield ratio (YR)of not less than 60%, a strength-ductility balance (tensilestrength×total elongation) of not less than 2,000 (kgf/mm².%), anenlargement ratio (d/d_(o)) of not less than 1.1, and a uniformelongation of not less than 10%, which comprises conducting afinish-rolling of a slab prepared by casting a steel consistingessentially of 0.15 to less than 0.30% by weight of C, 2.0 to 3.0% byweight of Si, 0.5 to 3.0% by weight of Mn, more than 2.5 to 6.0% byweight of Si and Mn in total, not more than 0.02% by weight of P, notmore than 0.01% by weight of S, and 0.005 to 0.10% by weight of Al, and0.0005 to 0.01% by weight of Ca or 0.005 to 0.05% by weight of REM, thebalance being Fe and inevitable elements, being composed of three phasesof ferrite, bainite and retained austenite as microstructure, and havinga ferrite grain size (d_(F)) of not more than 5 μm, a ratio (V_(F)/d_(F)) of ferrite volume fraction (V_(F)) to ferrite grain size (d_(F))of not less than 7, and a volume fraction of retained austenite having agrain size of not more than 2 μm being not less than 5%, at an endtemperature of not less than Ar₃ -50° C., at an entire draft of not lessthan 80% and an ultimate pass strain speed of not less than 30/second,conducting cooling at a hot run table down to a temperature T₁ in arange of not more than Ar₃ to more than Ar.sub. 1, at a rate of lessthan 30° C./second and from T₁ downwards at a rate of not less than 30°C./second, and conducting coiling at a temperature of more than 350° C.to 500° C.
 11. A process for producing a high yield ratio, hot rolledhigh strength steel sheet having an excellent formability and also ayield ratio (YR) of not less than 60%, a strength-ductility balance(tensile strength×total elongation) of not less than 2,000 (kgf/mm².%),an enlargement ratio (d/d_(o)) of not less than 1.1, and a uniformelongation of not less than 10%, which comprises conducting afinish-rolling of a slab prepared by casting a steel consistingessentially of 0.15 to less than 0.30% by weight of C, 2.0 to 3.0% byweight of Si, 0.5 to 3.0% by weight of Mn, more than 2.5 to 6.0% byweight of Si and Mn in total, not more than 0.02% by weight of P, notmore than 0.01% by weight of S, and 0.005 to 0.10% by weight of Al, thebalance consisting essentially of Fe, being composed of three phases offerrite, bainite and retained austenite as microstructure and having aferrite grain size (d_(F)) of not more than 5 μm, a ratio (V_(F) /d_(F))of ferrite volume fraction (V_(F)) to ferrite grain size (d_(F)) of notless than 7, and a volume fraction of retained austenite having a grainsize of not more than 2 μm being not less than 5%, at an end temperatureof not less than Ar₃ -50° C., at an entire draft of not less than 80%and an ultimate pass strain speed of not less than 30/second, conductingcooling at a hot run table down to a temperature T₁ in a range of notmore than Ar₃ to more than Ar₁ at a rate of not less than 30° C./second,from T₁ downwards to a temperature T₂ in a range of not more than T₁ tomore than Ar₁ at a rate of less than 30° C./second, and furthermore fromT₂ downwards at a rate of not less than 30° C./second, and conductingcoiling at a temperature of more than 350° C. to 500° C.
 12. A processfor producing a high yield ratio, hot rolled high strength steel sheethaving an excellent formability and also a yield ratio (YR) of not lessthan 60%, a strength-ductility balance (tensile strength×totalelongation) of not less than 2,000 (kgf/mm².%), an enlargement ratio(d/d_(o)) of not less than 1.1, and a uniform elongation of not lessthan 10%, which comprises conducting a finish-rolling of a slab preparedby casting a steel consisting essentially of 0.15 to less than 0.30% byweight of C, 2.0 to 3.0% by weight of Si, 0.5 to 3.0% by weight of Mn,more than 2.5 to 6.0% by weight of Si and Mn in total, not more than0.02% by weight of P, not more than 0.01% by weight of S, and 0.005 to0.10% by weight of Al, and 0.0005 to 0.01% by weight of Ca or 0.005 to0.05% by weight of REM, the balance being Fe and inevitable elements,being composed of three phases of ferrite, bainite and retainedaustenite as microstructure, and having a ferrite grain size (d_(F)) ofnot more than 5 μm, a ratio (V_(F) /d_(F)) of ferrite volume fraction(V_(F)) to ferrite grain size (d_(F)) of not less than 7, and a volumefraction of retained austenite having a grain size of not more than 2 μmbeing not less than 5%, at an end temperature of not less than Ar₃ -50°C. at an entire draft of not less than 80%, and an ultimate pass strainspeed of not less than 30/second, conducting cooling at a hot run tabledown to a temperature T₁ in a range of not more than Ar₃ to more thanAr₁ at a rate of not less than 30° C./second, from T₁ downwards to atemperature T₂ in a range of not more than T₁ to more than Ar₁ at a rateof less than 30° C./second, and furthermore from T₂ downwards at a rateof not less than 30° C./second, and conducting coiling at a temperatureof more than 350° C. to 500° C.
 13. A process for producing a high yieldratio, hot rolled high strength steel sheet excellent in bothformability and spot weldability according to any one of claims (1) to(6), wherein the hot finish-rolling initiation temperature of the steelis not more than Ar₃ +100° C.
 14. A process for producing a high yieldratio, hot rolled high strength steel sheet excellent in bothformability and spot weldability according to any one of claims (1) to(6), wherein after the coiling the steel sheet is cooled to 200° C. orless at a cooling speed of not less than 30° C./hour.
 15. A process forproducing a high yield ratio, hot rolled high strength steel sheetexcellent in formability according to any one of claims (7) to (12),wherein the hot finish-rolling initiation temperature of the steel isnot more than Ar₃ +100° C.
 16. A process for producing a high yieldratio, hot rolled high strength steel sheet excellent in formabilityaccording to any one of claims (7) to (12), wherein after the coilingthe steel sheet is cooled to 200° C. or less at a cooling speed of notless than 30° C./hour.